A high-dielectric constant ceramic composite and ceramic capacitor elements

Abstract: A high-dielectric constant ceramic composite is provided in which an appropriate amount of glass component is added to a composite of a lead based perovskite having a low temperature coefficient in the dielectric constant thereof. The added glass component serves to maintain the satisfactory properties of the ceramic composite even after various types of reliability tests. The glass component includes B₂O₃, SiO₂, Al₂O₃, BaO, and MgO.

Description: This invention relates to a high-dielectric constant ceramic composite, and more particularly to a high-dielectric constant ceramic composite that can be satisfactorily employed as a dielectric material in a multilayer ceramic capacitor. A dielectric material for use in ceramic capacitors is required to have electrical properties such as a high dielectric constant, a low temperature coefficient in the dielectric constant, and a small dielectric loss. Further, the required electrical properties include a dielectric constant which is less dependent on a biased electric field, and a large value of the product of its capacitance and the resistance, known as the CR value. To evaluate the reliability of a dielectric material, reliability tests such as a high-temperature load test, a high-humidity load test, and a heat-cycle test are performed. In the process of manufacturing multilayer ceramic capacitors, the electrode materials and dielectric materials thereof are fired simultaneously. Thus, the firing temperature of the dielectric materials is required to be lower than 1100 °C. This low firing temperature allows the use of Ag, which are less costly than Pt, Pd and the like, as the electrode material. For this reason, dielectric materials of the BaTiO₃ group have been widely used. However, in recent years, dielectric materials other than BaTiO₃, e.g., lead-based dielectric materials, which are called relaxors and represented by a general expression of Pb(B₁, B₂)O₃, have become of major interest. The lead-based dielectric materials serve to lower the firing temperature required for the process of manufacturing dielectrics of multilayer ceramic capacitors. Further, the materials serve to improve the electrical and mechanical properties of the dielectrics employed in the capacitors. For this reason, there have been disclosed several relaxor, such as the lead iron niobate group disclosed in Japanese Laid Open Patent (J. L. O. P.) No. 57-57204, the magnesium-lead niobate group disclosed in J. L. O. P. No. 55-51758, and the lead magnesium tungstate group disclosed in J. L. O. P. No. 52-21699. Further, the inventors of the present invention developed and disclosed the zinc-lead niobate group in J. L. O. P.s. No. 61-101460 and No. 61-155245. As described above, various sorts of relaxor have been developed. However, the durability thereof under severe environmental conditions (e.g. under a high-temperature load test) has not been sufficient. Moreover, when these relaxor are employed in the production of multilayer ceramic capacitors, the product of the capacitance and resistance is reduced during the high-temperature load test. The inventors of this invention have disclosed a combined material of BaTiO₃ and Pb(Zn 1/3 Nb 2/3 )O₃ as a high dielectric constant ceramic composite. The composite can be fired at a low temperature and has a low temperature coefficient of the dielectric constant. This material has been disclosed in Japanese Patent Application No. 63-156062. In the view of the high-humidity load test results alone, another improved composite has been disclosed in Japanese Patent Application No. 62-254310. Specifically, glass of Bi₂O₃-SiO₂-B₂O₃-Ag₂O was added to a composite of Pb(Zn 1/3 Nb 2/3 )O₃-Pb(Mg 1/3 -Nb 2/3 )O₃-PbTiO₃. However, in this disclosure the heat-cycle test was not taken into consideration. As described above, a satisfactory high-dielectric constant ceramic composite has not been obtained, which exhibits a low temperature coefficient of the dielectric constant and a small change in its properties under a heat-cycle test and a high-humidity load test. Accordingly, one object of the present invention is to provide a high-dielectric constant ceramic composite having a low temperature coefficient in the dielectric constant and a small change in its properties under a heat-cycle test and a high-humidity load test. More particularly, the object of this invention is to provide a high-dielectric constant ceramic composite having satisfactory properties suitable for manufacturing multilayer ceramic capacitors. In accordance with the present invention there is provided a high-dielectric constant ceramic composite comprising: at least one lead-based perovskite compound as a major relaxor component, and a glass component added to said relaxor component, characterised in that the glass component is derived from constituents which include 10-80 weight % B₂O₃, 10-60 weight % SiO₂, 1-20 weight % Al₂O₃, 1-20 weight % BaO, 1-20 weight % MgO, and optionally one or more of the following constituents: PbO, CaO, SrO, ZnO, TiO₂ or Nb₂O₅, wherein the total amount of constituents in the glass component is 100 weight %. Further preferred features of the invention are to be found in the sub claims. A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a ternary phase diagram illustrating an example of a major component of the dielectric material in a first embodiment according to the present invention; Figure 2 is a ternary phase diagram illustrating another example of a major component of the dielectric material in the first embodiment according to the present invention; Figure 3 is a ternary phase diagram illustrating still another example of a major component of the dielectric material in the first embodiment according to the present invention; and Figure 4 is a ternary phase diagram illustrating an example of a major component of the dielectric material in the second embodiment according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to Figure 1 thereof, a first embodiment of this invention will be described. A first embodiment according to the present invention is high-dielectric constant ceramic composite which can be obtained in the following manner. Specifically, a glass component, which includes at least one of silicon and boron, is added to a relaxor component having a lead-based perovskite structure as a major component. The glass component includes the following constituents in the weight percentages given below: B₂O₃ of 10 wt% to 80 wt% SiO₂ of 10 wt% to 60 wt% Aℓ₂O₃ of 1 wt% to 20 wt% BaO of 1 wt% to 20 wt% MgO of 1 wt% to 20 wt%, wherein the total amount of the constituents equals 100 wt%. Here, the amount of the glass component to be added to the relaxor component preferably must be 1 wt% at a maximum. Moreover, the inventors of the present invention have studied and discovered a highly reliable ceramic composite. Specifically, the composite has significantly improved the life performance under the high-temperature and load humidity tests. This can be achieved by adding a particularly prepared glass component to the relaxor component by a prescribed amount. The glass component includes Aℓ₂O₃, BaO, SrO, CaO and MgO of 20 wt% at a maximum. Further, the thus discovered composite can be manufactured by firing at lower temperatures. Moreover, the composite exhibits a high withstand voltage and a dielectric constant less dependent on the D.C. biased electric field. Further, the glass component may also include lead by 50 wt% at a maximaum. However, when the content of lead is more than 50 wt%, the composite deteriorates in durability under the high-humidity load test. When the content of lead is less than 35 wt%, the composite improves its reliability. Moreover, when even a small amount of lead, i.e., about 1 wt% is added to the glass component, the reliability of the composite can be significantly improved. In United States Patent 4,082,906, a glass component is added to a BaTiO₃-group material in order to lower the firing temperature. However, the glass component inevitably deteriorates the durability of the composite disclosed in this patent under the high-temperature load test. Such durability can be improved by use of the relaxor component having a lead-based perovskite structure as a major component. This composite of this invention has B₂O₃ of 10 wt% to 80 wt%, and SiO₂ of 10 wt% to 60 wt%. This is because if the content of the B₂O₃ less than 10 wt% or more than 80 wt%, the composite deteriorates in durability under the high-temperature load life test. Further, if the content of SiO₂ is less than 10 wt%, the durability under the high-temperature load life test is not improved. If the content of the SiO₂ is more than 60 wt%, it would be difficult to form glass at lower temperatures. Moreover, the respective contents of Aℓ₂O₃, BaO, SrO, CaO and MgO must be determined to be 20 wt% at a maximum. This is because if these contents are more than 20 wt%, the durability under high-temperature load life test would not be improved. The above-described SrO and CaO are not essential. However, if the SrO and CaO are added by about 0.1 wt%, better durability can be obtained. Further, the respective contents of Aℓ₂O₃, BaO and MgO must be determined to be 2 wt% at a minimum. This is because, if the contents are less than 2 wt%, the durability under the high-temperature load life test will not be improved. When even a small amount of the glass component is added to the relaxor component, this improves the properties of the composite of this invention. For example, when a glass component of 0.01 wt% is added to the relaxor component, increased durability of the composite and low firing temperature can be achieved. However, when the content of the glass component is more than 1.0 wt%, the dielectric constant of the composite is significantly lowered. Further, the dielectric loss of the composite will be increased. Thus, the content of the glass component preferably must be 1.0 wt% at a maximum so as to maintain the advantageous properties of the relaxor component. This invention relates to the advantage per se which are obtained by adding the above-described glass component. Thus, this invention is not limited to a certain particular technique in which the glass component is added. For example, the glass component may be added in the form of glass or in the form of the oxides of the components. In this invention, various types of relaxor components may be employed. For example, there can be cited relaxor components including the following basic components, such as Pb(Mg 1/3 Nb 2/3 )O₃, Pb(Zn 1/3 Nb 2/3 )O₃, PbTiO₃, PbZrO₃, Pb(Fe 1/2 Nb 1/2 )O₃, Pb(Fe 2/3 W 1/3 )O₃, Pb(Mg 1/2 W 1/2 )O₃ and Pb(Ni 1/3 Nb 2/3 )O₃. Naturally, a complex group of these basic components may be employed. Further, the relaxor component may include various types of additives such as MnO₂, Co₂O₃, La₂O₃, Sb₂O₃, NiO, ZrO₂ and Y₂O₃ by an amount of 0.5 wt% at a maximum. Here, assume that a material which includes at least 24e of the above-described Pb(Mg 1/3 Nb 2/3 )O₃ and Pb(Zn 1/3 Nb 2/3 )O₃ by the amount of 50 mol% at a minimum is employed as the relaxor component. This results in a satisfactory composite having a high dielectric constant and excellent life performance. In this case, the dielectric constant and insulation resistance of the material can be more significantly enhanced by substituting at least one of Ba, Sr and Ca for Pb atoms of 1 to 35 mol%. More preferable relaxor component can be obtained in the following manner. Specifically, a part of the Pb (later described) is substituted by at least one of Ba and Sr by about 1 mol% to about 35 mol%. Here, the above-described part of the Pb is defined as follows. First, assume that the perovskite compound, which is a major component of the relaxer, is represented by the general expression of xPb(Zn 1/3 Nb 2/3 )O₃ - yPb(Mg 1/3 Nb 2/3 )O₃ - zPbTiO₃. Three components of this expression are shown as apexes x, y and z in a prescribed ternary phase diagram of Figure 1. The above-described part of the Pb is limited to the area surrounded by lines a-b, b-c, c-d and d-a, including those points on all the lines except line a-b. In Figure 1, the respective points are determined as follows, a (x = 0.50, y = 0,00, and z = 0.50), b (x = 1.00, y = 0.00, and z = 0.00), c (x = 0.20, y = 0.80, and z = 0.00), and d (x = 0.05, y = 0.90, and z = 0.05). As described above, the basic composite of this embodiment is limited to the area surrounded by the lines connected to the points a, b, c and d, as shown in Figure 1. This is because when the basic composite is presnet outside the area, e.g., beyond the line a-d, the firing temperature is required to be 1100 °C or higher. Further, the insulation resistance of the composite is lowered, and thus the product of the capacitance and the resistrance thereof is also lowered. Moreover, when the basic composite is present beyond the line c-d, the dielectric constant thereof is significantly lowered. This is because the Cuire temperature thereof is originally in the region of normal temperature, and the substitution by the Me component can easily occur. Further, when the point d changes to a point d₁ (where x = 0.10, y = 0.80 and z = 0.10), the area inside the line c-d₁ is more preferable, as shown in Figure 1. When a small amount of lead magnesium niobate is added to the composite of this embodiment, the advantages thereof can be obtained. However, amounts greater than 1 mol% thereof are preferably in a practical application. In order to increase the CR value (the product of capacitance and resistance), Zinc-lead niobate of 15 mol% must be included. More preferably, 20 mol% or more thereof must be included. When more than 20 mol% thereof is included, the dielectric loss will be significantly reduced. Further, assume that the points c and d change respectively to points c₁, c₂, d₂, and d₃. Specifically, in Figure 1, the respective points are defined as follows: c₁ (x = 0.40, y = 0.60, and z = 0.00), c₂ (x = 0.45, y = 0.55, and z = 0.00), d₂ (x = 0.15, y = 0.70, and z = 0.15), and d₃ (x = 0.20, y = 0.60, and z = 0.20). In view of the CR value, T.C.C. (temperature coefficient of dielectric constant) and sintering properties, the basic composite must be present inside the line c₁-d₂, preferably inside the line c₂-d₂ and more preferably inside the line c₂-d₃. However, in view of the dielectric constant alone, satisfactory, properties can be obtained even when the basic composite is present in the area defined by the lines connected respectively to the points a, b, c and d. More preferably, the basic composite includes yPb(Mg 1/3 Nb 2/3 )O₃ as of y ≧ 0.01, zPbTiO₃ as of z ≧ 0.01, Pb(Mg 1/3 Nb 2/3 )O₃, and PbTiO₃. Further, the elements Ba and Sr are necessary to form the perovskite structure of the above-described general expression. When the contents of the Ba and Sr are less than 1 mol%, a pyrochroite structure is inevitably mixed in the perovskite structure. As a result, a high dielectric constant and a high insulation resistance cannot be obtained. However, if the contents of the Ba and Sr are more than 35 mol%, the dielectric constant would decrease to about 1000 or less. Thus, the contents of Ba and Sr must be determined to be 0.01 < α < 0.35 where the substitution amount in the Me component is represented by (Pb 1-α Me α ). In addition, the basic composite of this embodiment can be obtained in the following manner. Specifically, a part of the Pb (later described) is substituted by at least one of Ba and Sr by about 1 mol% to about 30 mol%. Here, the above-described part of the Pb is defined by using the general expression of xPb(Zn 1/3 Nb 2/3 )O₃ - yPb(Mg 1/3 Nb 2/3 )O₃ - zPb(Ni 1/3 Nb 2/3 )O₃. Three components of this expression are shown as apexes x, y and z in a prescribed ternary phase diagram of Figure 2. The above-described part of the Pb is limited in the area surrounded by lines a-b, b-c and c-a except for those points on the lines. In this chart, the respective points are determined as follows, a (x = 0.50, y = 0.00, and z = 0.50), b (x = 1.00, y = 0.00, and z = 0.00), and c (x = 0.10, y = 0.90, and z = 0.00). Further, the basic composite of this embodiment can be obtained in a similar manner as above, except for the following limitations. Specifically, the general expression to be applied is (1-x) (Pb 1-a-b Ba a Sr b )-{(Zn 1/3 Nb 2/3 ) 1-a-d (Mg 1/3 Nb 2/3 )Tid}O₃. xBaTiO₃, where the following conditions must be satisfied: 0 ≦ a ≦ 0.35, 0 ≦ b ≦ 0.35, 0.01 ≦ a+b ≦ 0.35, 0 < c ≦ 0.9, 0 < d ≦ 0.5, and 0.3 ≦ x ≦ 0.5. Moreover, the basic composite of this embodiment can be obtained in a similar manner as above, except with the following limitations. Specifically, the general expression to be applied is x.Pb(Zn 1/3 Nb 2/3 )O₃ - y.Pb(Mg 1/3 Nb 2/3 )O₃. z.PbTiO₃, where the following conditions must be satisfied as shown in Figure 3: a (x = 0.60, y = 0.40, and z = 0.00), b (x = 0.60, y = 0.05, and z = 0.35), c) (x = 0.45, y = 0.05, and z = 0.50), d (x = 0.01, y = 0.49, and z = 0.50), e (x = 0.01, y = 0.85, and z = 0.14), and f (x = 0.15, y = 0.85, and z = 0.00). Further, in this case, the Pb is substituted by Ca of about 2 mol% to about 30 mol%. In the process of manufacturing a multilayer ceramic capacitor element, a binder and a solvent are added to the above-described material powder so as to form a slurry. The slurry is cast into green sheets by the use of a caster. Thereafter, inner electrodes are printed on the green sheets, respectively. Next, the prescribed number of the sheets are multi-layered, laminated using pressure, and fired. In this case, the dielectric materials of this invention can be fired at a relatively low temperature. Thus, less costly materials such as Ag major (Ag of about 80 to 50%, Pb of about 20 to 50%) can be used as a material of the inner electrodes. Moreover, the composite of this invention can be advantageously used as a thick film dielectric paste. The paste is printed on a printed circuit board, and then fired at a low temperature. The ceramic composite of this invention has a high dielectric constant and satisfactory temperature characteristics, and exhibits a sufficiently large CR value even at high temperatures. Thus, a highly reliable ceramic composite can be obtained. Moreover, the composite of this invention has a dielectric constant substantially independent of a biased electric field applied thereto. For example, a material whose dielectric constant varies only about 10% in a field of 2 KV/mm can be obtained. Thus, it is advantageous to use the composite of this invention as a material for high-voltage applications. Further, the composite of this invention has a small dielectric loss. Thus, the composite can be advantageously applied to alternating-current circuits and high-frequency circuits. In addition, the composite of this invention has a dielectric constant that exhibits small temperature dependence. Thus, the composite can be used for a minute displacement element which utilizes piezoelectric effects and electrostriction effects. In this case, less temperature-dependent characteristics can also be obtained. Hereinafter, examples in this embodiment will be described with reference to Tables 1 and 2. First, oxides of Pb, Ba, Sr, Zn, Nb and Mg were mixed in a prescribed proportion. The thus obtained source material was calcined at about 700 °C to about 850 °C. Thereafter, the thus fired material was ground by the use of a ball mill and reduced to powder. The powder was dried, and then glass powder shown in Table 1 was added to the dried powder. The added glass powder was ground by use of the ball mill and reduced to powder. The thus obtained powder was dried. Thereafter, a binder and an organic solvent were added to the dried powder, and a slurry was obtained. Next, the thus obtained slurry was cast into a green sheet about 30 µm thick by the use of a doctor blade-type caster. Next, electrode paste of 70Ag/30Pd was printed on the green sheet in a prescribed pattern. Twenty green sheets, each having the prescribed pattern, were prepared. Such twenty green sheets were stacked and laminated using pressure so as to form a multilayer structure. Then, the formed multilayer structure was cut into a prescribed shape. Thereafter, binder burn out of the multilayer ceramic capacitor was carried out and it's fired at about 1020 °C for about 2 hours. Next, Ag paste was applied to the terminations of the multilayer ceramic capacitor and fired. Table 2 shows electrical properties of multilayer ceramic capacitors manufactured as described above. As shown in Table 2, fifteen different types of examples designated by No. 1 through 15, and a comparison example No. 1, were prepared. Each example included 20 pieces of the capacitors for measuring electrical properties thereof, and 200 pieces of the capacitors for reliability tests. Each example was prepared employing materials such as Pb, Ba, Sr, Zn, Nb and Mg in different constitutive proportion. Further, different types of glass components designated by No. 1 through 5 of Table 1 were added respectively to the examples. The measurements of the capacitors were obtained in the following manner, and the averaged values of each 20-piece example are shown in Table 2. Specifically, the insulation resistance of the capacitors was measured at 25 °C by the use of an insulation resistance-meter after the application of about 25 V D.C. for about 2 minutes. The dielectric loss and capacitance of the capacitors were measured by the use of a digital LCR-meter under the condition in which 1KHz of 1 Vrms was applied at 25 °C. The dielectric constants of the capacitors were calculated on the basis of these measurements. The CR value (the product of the capacitance and resistance) was obtained from the result of (dielectric constant at 25 °C) × (insulation resistance at 25 °C) × (dielectric constant under vacuum). The temperature coefficient of the capacitance (T.C.C.) was represented by the changes of the capacitance at -25 °C and a +85 °C respectively in comparison with that at 25 °C. The reliability tests such as the high-temperature load life test and the withstanding voltage test were performed in the following manner. Specifically, the capacitors were placed in a chamber and exposed to an atmosphere of 120 °C for about 500 hours while a voltage of about 100 V D.C. was applied thereto. Thereafter, the CR values of the capacitors were measured. The capacitors whose CR values were less than 1000 ΩF were counted as failures. Further, a voltage of about 300 V D.C. was applied to the capacitors at 25 °C for about 1000 hours. Thereafter, the capacitors were tested to determine whether they had broken down. Moreover, in this embodiment, the glass component may become a liquid phase during the manufacturing process. Thus, the sintering of the composite can be performed at lower temperatures than that of the conventional composite. Specifically, in the conventional technique, the composite must be sintered at temperatures between about 1050 °C and about 1150 °C. Otherwise, the composite will not be sufficiently sintered. According to the present invention, the sintering of the composite can be performed at temperatures between about 950 °C and about 1100 °C. Further, the composite of this invention can be steadily sintered without precise control of the sintering temperature. This is advantageous to mass-produce multilayer ceramic capacitors. Further, when the composite of this invention is employed in manufacturing multilayer ceramic capacitors, less costly Ag/Pd electrode paste can be applied. In addition, the composite of this invention is hardly reduced even when being exposed to a reducing atmosphere of N₂ and the like. Thus, the composite can be applied to the process of manufacturing multilayer ceramic capacitors using electrode paste of base metals, such as Ni, Cu and the like. This is also advantageous to reduce the production cost. In the conventional technique of sintering a ceramic composite of the Pb group, a dense sagger or a setter has been used. The sagger or setter, either of which is made of expensive MgO or Aℓ₂O₃, serves to suppress the evaporation of the Pb. However, according to the present invention, such sintering can be performed by the use of a less costly porous sheath or setter. Next, a second embodiment according to the present invention will be described. In this embodiment, a high-dielectric constant ceramic composite is provided which comprises a component having a perovskite compound of the Pb group as a major component. In view of the micro-structure of the sintered composite of this embodiment, an amorphous phase comprising Pb and Si is formed at a triple point. In this case, a triple point is defined as a point at which three particles or more are collected. The inventors of this invention discovered that the composite having the above-described triple point exhibits a significantly enhanced reliability under the high temperature and humidity load life tests. Further, the composite in this embodiment has a high withstanding voltage and a dielectric constant less dependent on the DC-biased field. In this embodiment, various types of relaxor components may be employed. For example, there can be cited relaxer components including basic components such as Pb(Mg 1/3 Nb 2/3 )O₃, Pb(Zn 1/3 Nb 2/3 )O₃, PbTiO₃, PbZrO₃, Pb(Fe 1/2 Nb 1/2 )O₃, Pb(Fe 2/3 W 1/3 )O₃, Pb(Mg 1/2 W 1/2 )O₃ and Pb(Ni 1/3 Nb 2/3 )O₃. Naturally, a complex group of these basic components may be employed. Further, relaxer components may include various type of additives such as MnO₂, Co₂O₃, La₂O₃, Sb₂O₃, NiO, ZrO₂ and Y₂O₃ by the amount of 0.5 wt% at a maximum. Here, assume that a composite, which includes at least one of the above-described Pb(Mg 1/3 Nb 2/3 )O₃ and Pb(Zn 1/3 Nb 2/3 )O₃ by the amount of 50 mol% at a minimum, is employed as the relaxor component. This results in a satisfactory material having a high dielectric constant and excellent life performance. In this csse, the dielectric constant and insulation resistance of the material can be more significantly enhanced by substituting at least one of Ba, Sr and Ca for Pb atoms of 1 to 35 mol%. More preferable relaxor component can be obtained in the following manner. Specifically, a part of the Pb (later described) is substituted by at least one of Ba and Sr by about 1 mol% to about 35 mol%. Here, the above-described part of the Pb is defined as follows. First, assume that the perovskite compound, which is a major component of the relaxer, is represented by the general expression of xPb(Zn 1/3 Nb 2/3 )O₃ - yPb(Mg 1/3 Nb 2/3 )O₃ - zPbTiO₃. Three components of this expression are shown as apexes x, y and z in a prescribed ternary phase diagram of Figure 4. The above-described part of the Pb is limited in the area surrounded by lines a-b, b-c, c-d and d-a including those points on all the lines except line a-b. In this chart, the respective points are determined as follows, a (x = 0.50, y = 0.00, and z = 0.50), b (x = 1.00, y = 0.00, and z = 0.00), c (x = 0.20, y = 0.80, and z = 0.00), and d (x = 0.05, y = 0.90, and z = 0.05). As described above, the basic composite of this embodiment is limited to the area surrounded by the lines connected to the points a, b, c and d, as shown in Figure 4. This is because when the basic composite is present outside the area, e.g., beyond the line a-d, the firing temperature is required to be 1100 °C or higher. Further, the insulation resistance of the composite is lowered, and thus the product of the capacitance and the resistance thereof is also lowered. Moreover, when the basic composite is present beyond the line c-d, the dielectric constant thereof is significantly lowered. This is because the Curie temperature thereof is inherently in the region of normal temperature, and the substitution by the Me component can easily occur. Further, when the point d changes to a point d₁ (where x=0.10, y=0.80 and z=0.10) as shown in Figure 4, the area inside the line c-d₁ is more preferable for the basic composite to exist. As described above, in this embodiment, an amorphous phase is formed at a triple point. This is performed in the following manner. Specifically, a glass component whose softening point is lower than the sintering temperature of the ceramic composite is mixed with the materials of the ceramic composite. Thereafter, the thus obtained mixture is heated at a relatively lower heating rate. For example, the heating is performed at a rate of about 15 °C to about 50 °C per hour starting from a temperature of about 300 °C below a prescribed firing temperature. The heating is continued substantially at the prescribed firing temperature for about 1 to 3 hours. As a result, the glass component can be formed at the triple point. Thus, the glass phase can be uniformly distributed throughout the entire structure. After the completion of firing, the temperature is decreased rapidly in the following manner. Specifically, the mixture is cooled down at a rate of about 300 °C per hour from the prescribed firing temperature to about 600 °C. This rapid cool-down is indispensable to form the amorphous phase at the triple point. If the temperature decrease is slow, a reaction would take place between the amorphous phase and the dielectric particles, potentially resulting in adverse effects upon dielectric properties. The amorphous phase comprises components such as PbO, B₂O₃ and SiO₂ as major components, and other components such as ZrO₂, TiO₂ and Aℓ₂O₃ which are added to PbO and SiO₂. Hereinafter, examples of this embodiment will be described with reference to Tables 3, 3(2) and 4. First, oxides of Pb, Ba, Sr, Zn, Nb and Mg were mixed in a prescribed proportion. The thus obtained source material was calcined at temperature between about 700 °C and about 850 °C. Thereafter, the thus fired material was ground by the use of a ball mill and reduced to powder. The powder was dried, and then glass powder shown in Table 3 was added to the dried powder. The resultant mixture powder was further ground by use of the ball mill. The thus obtained powder was dried. Thereafter, a binder and an organic solvent were added to the dried powder, and a slurry was obtained. Next, the slurry was cast into a green sheet about 30 µm thick using a doctor blade. Next, electrode paste of 70Ag/30Pd was printed on the thus obtained green sheets in a prescribed pattern. Twenty green sheets with the prescribed pattern were prepared. Such twenty green sheets were stacked and laminated using pressure. The thus obtained multilayer structure was cut into a prescribed shape. Thereafter, binder burn out of the multilayer ceramic capacitor was carried out and it's fired in the following manner. Specifically, the firing temperature was increased from the room temperature up to about 800 °C at a rate of about 100 °C per hour. Further, the firing temperature was increased up to about 1050 °C at a rate of about 25 °C per hour. The multilayer ceramic capacitor was then fired at about 1050 °C for about 2 hours. Next, the temperature was decreased from about 1050 °C to about 700 °C at a rate of about 500 °C per hour, and thereafter at a rate of about 100 °C per hour. After the completion of firing, Ag paste was applied to the multilayer ceramic capacitor so as to form terminations. The electrical properties of the thus obtained capacitors were measured in the following manner. Specifically, the dielectric loss and capacitance of the capacitors were measured by the use of a digital LCR meter. The measurement was performed with a voltage of about 1 KHz and 1 Vrms applied to the capacitors at 25 °C. The dielectric loss was then calculated on the basis of the measurements. The durability tests of the capacitors were performed in the following manner. Specifically 22 different types of examples and 9 different types of comparison examples were prepared. Each example included 1000 capacitors, and each comparison example included 1000 capacitors. In the case of the high-humidity load life test, the capacitors were placed in a chamber, and exposed to an atmosphere of about 55 °C and 95% RH. Further, a voltage of 25 V D.C. was continuously applied to the capacitors for about 500 hours. In the case of the high-temperature load life test, the capacitors were placed in the thermostat, and exposed to an atmosphere of about 120 °C. Further, a voltage of 150 V D.C. was continuously applied to the capacitors for about 500 hours. After 500 hours the capacitors were examined as to whether they had broken down, i.e., short-circuited or significantly deteriorated in insulated resistance. The broken-down capacitors were counted as defectives in the respective test. In Table 4, such defectives are represented by percent. As can be seen from Table 4, capacitors containing the composite of this embodiment can maintain their satisfactory properties thereof even after the above-described durability tests. As a result, the reliability of the composite in this embodiment can be significantly enhanced. Table 3 shows the composition of triple points in sintered bodies. As can be seen from Table 3, in addition to lead, Ti, Nb, Zn, etc., are included, which can be regarded as solid solutions derived from the relaxor component. The solid solutions can not be avoided, and the content thereof is usually about 1 wt% or more. Further, lead of the triple points must be 50 wt% at a maximum, and more preferably 35 wt% at a maximum. When the lead of the triple points is excessive, the composite deteriorates in reliability under the high-humidity load life test. As a result, the reliability of the composite is inevitably lowered.

Claims: A high-dielectric constant ceramic composite comprising: at least one lead-based perovskite compound as a major relaxor component, and a glass component added to said relaxor component, characterised in that the glass component is derived from constituents which include 10-80 weight % B₂O₃, 10-60 weight % SiO₂, 1-20 weight % Al₂O₃, 1-20 weight % BaO, 1-20 weight % MgO, and optionally one or more of the following constituents: PbO, CaO, SrO, ZnO, TiO₂ or Nb₂O₅, wherein the total amount of constituents in the glass component is 100 weight %. A composite as claimed in claim 1 wherein the glass component further comprises: PbO in an amount not exceeding 50 weight %. A composite as claimed in either preceding claim, wherein the glass component further comprises SrO in an amount not exceeding 20 weight %. A composite as claimed in any preceding claim, wherein the glass component further includes CaO in an amount not exceeding 20 weight %. A composite as claimed in any preceding claim wherein the amount of the glass component added is between 0.005 weight % and 1.0 weight %. A composite as claimed in any preceding claim, wherein the relaxor component comprises Pb(Zn 1/3 Nb 2/3 ) O₃, and optionally one or more of the following additives: MnO₂, Co₂O₃, La₂O₃, Sb₂O₃, NiO, ZrO₂ and Y₂O₃. A composite as claimed in claim 6 wherein the relaxor component comprises at least 50 mol % of Pb(Mg 1/3 Nb 2/3 )O₃ and/or Pb(Zn 1/3 Nb 2/3 )O₃ and optionally one or more of PbTiO₃, PbZrO₃, Pb(Fe 1/2 Nb 1/2 )O₃, Pb(Fe 2/3 W 1/3 )O₃, Pb(Mg 1/2 W 1/2 )O₃ and Pb(Ni 1/3 Nb 2/3 )O₃. A composite as claimed in claim 7 wherein the lead is partially substituted by at least one of Ba, Sr and Ca. A composite as claimed in claim 8, wherein part of the lead of the perovskite compounds is substituted by Ba and/or Sr in an amount of 1 to 35 mol %, the perovskite compounds being represented by: x Pb(Zn₁ /3 Nb 2/3 )O₃ - y Pb(Mg 1/3 Nb 2/3 )O₃ - z PbTiO₃, the three components of which are shown as apexes x, y and z in a predetermined ternary phase diagram, and wherein the part of the lead is limited to the area surrounded by lines a-b, b-c, c-d, and d-a including those points on the lines except on the line a-b, said points a, b, c and d, being determined as follows: a(x= 0.50, y= 0.00, z= 0.50) b(x= 1.00, y= 0.00, z= 0.00) c(x= 0.20, y= 0.80, z= 0.00) d(x= 0.05, y= 0.90, z= 0.05). A multilayer capacitor constructed from dielectric material composite as claimed in any preceding claim. A capacitor as claimed in claim 10 wherein the glass contains PbO and has an amorphous phase comprising Pb and Si formed at triple points. A capacitor as claimed in claim 10 or 11, further comprising the perovskite compound BaTiO₃ as a major component of the ceramic composite.

Inventors: FURUKAWA OSAMU C O INTELLECTUA, HARADA MITSUO C O INTELLECTUAL, KANAI HIDEYUKI C O INTELLECTUA, YAMASHITA YOHACHI C O INTELLEC, FURUKAWA, OSAMU C/O INTELLECTUAL PROPERTY DIV., HARADA, MITSUO C/O INTELLECTUAL PROPERTY DIV., KANAI, HIDEYUKI C/O INTELLECTUAL PROPERTY DIV., YAMASHITA, YOHACHI C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1989
Main classification: H01B3
IPC codes: C03C3, C04B35, H01L41, H01G4, H01B3