Dielectric material and capacitor made from the same

Abstract: The dielectric material includes a pre-baked powder containing Pb(Mg 1/3 Nb 2/3 )O₃, BaTiO₃, and BaZrO₃, and lead oxides alone or lead oxides and copper oxides in combination. Thus, the dielectric material is sintered at low temperatures of 800-1000°C, and makes it possible to use inexpensive electrode materials such as copper, silver, and silver-palladium alloy for the electrode. The dielectric material also makes it possible to produce a ceramic condenser or a thin condenser having a stable capacitance irrespective of changes in temperature.

Description: 1. Field of the Invention: The present invention relates to a dielectric material that can be baked at low temperatures for a short period of time, and a condenser made from the dielectric material, thereby having excellent temperature characteristics, such as a dielectric constant varying within 10% of the base value measured at 20°C in the temperature range of -25°C to 85°C. 2. Description of the Prior Art: Ceramic condensers are comprised of a dielectric material and electrodes placed on both sides of the dielectric material. The dielectric material requires, as one of electric properties the temperature dependence of its dielectric constant which is defined by JIS, etc. Examples of the dielectric materials having excellent temperature characteristics of dielectric constant include barium titanate materials which are used for conventional ceramic condensers. These dielectric materials are usually baked in the manufacturing process of the ceramic condenser. However, the barium titanate materials should be baked at high temperatures of approximately 1300°C in the atmosphere. Electrode materials that can be used at such high temperatures during the baking process include only noble metals which have relatively high melting points, for example platinum, palladium, etc. The use of these expensive noble metals leads to the increase of the ceramic condenser prices. In order to obtain inexpensive ceramic condensers, the development of dielectric materials having lower baking temperatures and replacing barium titanium have been demanded. The proposed dielectric materials are ones made of perovskite type materials containing lead ion as a base, which have not only low baking temperatures, but provide excellent temperature characteristics of dielectric constant. Since these dielectric materials are sintered at these low temperatures, less expensive electrode materials having lower melting points, e.g., silver-palladium alloy can be used. Therefore, using these dielectric materials, and the electrode materials, ceramic condensers can be made at a lower cost. With the recent development of compact and very reliable electronic devices, a thick-film condenser has been demanded as a ceramic condenser instead of a conventional chip condenser as the number of circuits placed on a single plate is increased. The thick-film condenser structurally requires a dielectric material that can be baked not only at low temperatures, but for a short period of time. Therefore, a dielectric material that can be used for such a thick-film condenser with flat temperature characteristics will be increasingly required. However, the aforementioned perovskite type dielectric material must be sintered in an atmosphere of low partial oxygen pressure so as to avoid oxidation and also be maintained at a temperature of 1000°C or higher for several hours so as to obtain a solid sintered body with a high dielectric constant. When a thick-film condenser is manufactured using the above proposed materials, the dielectric material cannot be sufficiently sintered because a low-temperature and short-time sintering process is required for a thick-film condenser, thus failing in obtaining a desired characteristic of the condenser. If it is intended to sinter the dielectric material completely, the sintering temperature must be 1000°C or higher. At such high temperatures, as mentioned above, electrode materials may be oxidized, and therefore, the above-mentioned silver-palladium alloy, or copper that has a further lower melting point, cannot be used, thus preventing the manufacture of ceramic condensers at a lower cost. US-A-4712 156 discloses a dielectric material consisting of lead magnesium niobdate and barium titanate and copper oxide; This ferroelectric ceramic material has a high dielectric constant and shows a perovskite crystal phase. The objective of this invention is to provide a dielectric material which has the flat temperature characteristics of dielectric constant and is able to be sintered at a low temperature of 1000°C or lower in a short period of time. The another objective of this invention is to provide an inexpensive condenser using such a dielectric material. SUMMARY OF THE INVENTION The dielectric material of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, consists essentially of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula, and 1.0-25.0 mole % of lead oxides in which the amounts of the lead oxides are calculated as the amount of PbO: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O₃ +a ] xBa(Ti 1-b Zr b )O₃ wherein, x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.30, and 0≦b≦0.15. In a preferred embodiment, the dielectric porcelain composition is a mixture of the solid solution of BaTiO₃ and BaZrO₃ to Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a . In a preferred embodiment, the lead oxide is PbO, Pb₃O₄, or PbO₂. An embodiment of the dielectric material of this invention consists essentially of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula, and 1.0-25.0 mole % of lead oxides, and 1.0-15.0 mole % of copper oxides in which the amounts of the lead oxides, and the copper oxides are calculated as those of PbO, and CuO, respectively: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein, x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.25, and 0≦b≦0.15. In a preferred embodiment, the dielectric porcelain composition is a mixture of the solid solution of BaTiO₃ and BaZrO₃ to Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a . In a preferred embodiment, the lead oxide is PbO, Pb₃O₄, or PbO₂. In a preferred embodiment, the copper oxide is Cu₂O or CuO. Also, the condenser of this invention comprises a dielectric layer, and electrodes placed on both sides of the dielectric layer, wherein the dielectric layer is made up essentially of a dielectric material consisting of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula, and 1.0-25.0 mole % of lead oxides in which the amounts of the lead oxides are calculated as the amount of PbO: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein, x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.30, and 0≦b≦0.15. In a preferred embodiment, the electrode is made from copper, silver, or silver-palladium alloy. Another condenser according to an embodiment of this invention comprises a dielectric layer, and electrodes placed on both sides of the dielectric layer, wherein the dielectric layer is made up essentially of a dielectric material consisting of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula, and 1.0-25.0 mole % of lead oxides, and 1.0-15.0 mole % of copper oxides in which the amounts of the lead oxides, and the copper oxides are calculated as those of PbO, and CuO, respectively: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein, x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.25, and 0≦b≦0.15. In a preferred embodiment, the electrode is made from copper, silver, or silver-palladium alloy. Another condenser according to an embodiment of this invention comprises a ceramic base plate, a lower electrode, a dielectric layer, and an upper electrode, wherein the lower electrode, the dielectric layer, and the upper electrode are laminated in this order on the ceramic base plate, wherein the dielectric layer is made up essentially of a dielectric material consisting of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula, and 1.0-25.0 mole % of lead oxides in which the amounts of the lead oxides are calculated as the amount of PbO: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein, x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.30, and 0≦b≦0.15. In a preferred embodiment, the electrode is made from copper, silver, or silver-palladium alloy. Another condenser according to an embodiment of this invention comprises a ceramic base plate, a lower electrode, a dielectric layer, and an upper electrode, wherein the lower electrode, the dielectric layer, and the upper electrode are laminated in this order on the ceramic base plate, wherein the dielectric layer is made up essentially of a dielectric material consisting of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula, and 1.0-25.0 mole % of lead oxides, and 1.0-15.0 mole % of copper oxides in which the amounts of the lead oxides, and the copper oxides are calculated as those of PbO, and CuO, respectively: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein, x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.25, and 0≦b≦0.15. In a preferred embodiment, the electrode is made from copper, silver, or silver-palladium alloy. Thus, the invention described herein makes possible the objectives of (1) providing a dielectric material that can be baked at low temperatures of 1000°C or less for a short period of time; and (2) providing a condenser which is made from the dielectric material, and has excellent temperature characteristics, i.e., stable dielectric characteristics at different temperatures, for example a dielectric constant varying within 10% of the base value measured at 20°C in the temperature range of -25°C to 85°C. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a perspective view of the ceramic condenser illustrating one embodiment of this invention. Figure 2 is a top plan view of the thick-film condenser illustrating one embodiment of this invention. Figure 3 is a side cross-sectional view of the thick-film condenser illustrating one embodiment of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples illustrate the present invention in detail. Example 1 A Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a (hereinafter referred to as PMN) powder is prepared by pre-baking the combination of the prescribed amounts of MgO and Nb₂O₅ at 950°C for 5 hours in the atmosphere to obtain MgNb₂O₆, grinding the MgNb₂O₆ by a ball mill, then pre-baking the combination of the prescribed amounts of the ground MgNb₂O₆, PbO and CuO at 900°C for 2 hours in the atmosphere, and grinding the combination by a ball mill. On the other hand, a solid solution of BaTiO₃ and BaZrO₃ (hereinafter referred to as BTZ) is prepared by pre-baking the combination of the prescribed amounts of BaCO₃, TiO₂ and ZrO₂ at 1300°C for 2 hours in the atmosphere, and then grinding the combination by a ball mill. Then, the prescribed amounts of the resulting PMN powder, and BTZ powder are wet-blended by a ball mill, and dried. The dried mixture is pre-baked at 800°C for 2 hours in the atmosphere, ground by a ball mill, and dried. To this mixture PbO and CuO are added as sub-components with mixing by a mixing and grinding machine. To the powder mixture, 6 wt% polyvinylalcohol solution is added as a binder in the amount of 6 wt% of the mixture, and granulated by a 32 mesh sieve. The final mixture is molded under a pressure of 1000 kg/cm² in the form of a disk having about 13 mm diameter, and about 5 mm thickness. Then, the molding is allowed to stand at 600°C for 1 hour in the atmosphere to remove the binder, and baked at 900°C for 10 minutes under a nitrogen atmosphere. The baked molding is worked into a disk having 1 mm thickness, coated with an Ag paste on both sides as electrodes, and dried at 120°C in the atmosphere to form a single-plate condenser as shown in Figure 1. The condenser comprises of electrodes 1, 1′, and a dielectric Layer 2 as shown in Figure 1. Table 1 illustrates the composition of the major components and the sub-components contained in the dielectric materials of this example. Also, Table 2 illustrates the dielectric characteristics of the single-plate condenser, i.e., dielectric constant, tan δ (dielectric loss factor), and insulation resistance, and temperature variation percentage of the dielectric constant (a value measured at 20°C is the base value). The dielectric constant, and tan δ (dielectric loss factor) are measured under an electric field of 1 kHz and 1 V/mm. Also, the dielectric resistance is measured one minute after a 30 V is applied. As can be seen from Tables 1 and 2, the dielectric materials of this invention (samples Nos. 2-4, 6-11, 15-19, and 23) are sintered sufficiently, and have a lower temperature variation percentage, i.e., a dielectric constant varying within 10% of the base value measured at 20°C in the temperature range of -25°C to 85°C, and a high enough insulation resistance for practical uses even though it is baked at a low temperature of 900°C for a short period of time. Especially, the dielectric materials have improved temperature characteristics in high temperature areas because a suitable amount of Zr atom contained therein reduces its Curie point. On the other hand, control sample No. 5 is not sintered sufficiently at 900°C because of the lower amounts of PbO and CuO, i.e., the absence of PbO or CuO as the sub-component, and therefore provides an unacceptably lower dielectric constant. Although control sample No. 12 containing the excess amounts of PbO and CuO is sintered sufficiently, it has unsatisfactory temperature characteristics of dielectric constant, i.e., a higher temperature variation percentage, and increased dielectric loss which is not acceptable for practical uses. When a small amount of BTZ is added (control sample No. 1), the dielectric constant of the resulting dielectric material has higher temperature dependence derived from PMN. On the other hand, control sample No. 21 containing a large amount of BTZ is neither sintered sufficiently, nor provides excellent characteristics derived from PMN. Also, control sample No. 20 which contains a large amount of Zr does not provide good temperature characteristics because of severely reduced Curie point. Moreover, control sample No. 14 is not acceptable because of low dielectric loss factor derived from the excess amount of Cu contained in PMN. Example 2 Single-plate condensers are made by the same procedure as in Example 1 except that the powder mixture is prepared by adding as the sub-components the prescribed amounts of Pb₃O₄, PbO₂ and Cu₂O to the dielectric porcelain composition represented by the following formula: 0.60[Pb(Mg 1/3 Nb 2/3 )Cu 0.1 O 3.1 ] 0.40Ba(Ti 0.9 Zr 0.1 )O₃ The dielectric characteristics of the single-plate condensers, i.e., dielectric constant, dielectric loss factor, insulation resistance, and temperature variation percentage of the dielectric constant (a value measured at 20°C is the base value) are also measured in the same manner. Table 3 illustrates the amounts added of Pb₃O₄, PbO₂ and Cu₂O. Table 4 illustrates the dielectric characteristics, i.e., dielectric constant, tan δ (dielectric loss factor), and insulation resistance, and temperature variation percentage of the dielectric constant. The added amount of each Pb₃O₄ and PbO₂ are calculated as that of PbO, and the added amount of Cu₂O into that of CuO. As can be seen from Tables 3 and 4, the dielectric materials of this invention (samples Nos. 2-4, 6-11, 15-19, and 23) are sintered sufficiently, and have a lower temperature variation percentage, i.e., a dielectric constant varying within 10% of the base value measured at 20°C in the temperature range of -25°C to 85°C, and a high enough insulation resistance for practical uses even though it is baked at a low temperature of 900°C for a short period of time. When Cu₂O is used as the sub-component added in the dielectric composition of major components, almost the same results as using CuO in Example 1 are achieved because Cu₂O is oxidized to CuO under atmospheric baking conditions. Also, because Pb₃O₄ and PbO₂ are decomposed into PbO and oxygen at the baking temperature, the same results as when PbO is used in Example 1 are obtained. Example 3 To the dielectric composition of major components which is pre-baked and dried by the same procedure as in Example 1, there are added the prescribed amounts of PbO and CuO with wet-blending by a ball mill, and then dried. Then, a vehicle of a resin which contains ethyl cellulose as a major component dissolved in a solvent is added to the mixture of major component and sub-component, and kneaded by a three-stage roll to prepare a dielectric paste. On the other hand, copper paste is coated as a lower electrode 3 on an alumina base plate 4 having 96% purity, and dried as shown in Figures 2 and 3. Then, the aforementioned dielectric paste is coated in a 50-60 µm thickness on the lower electrode 3, and dried to form a dielectric layer 2. Moreover, the same copper paste as used for the lower electrode 3, is coated as an upper electrode 1 on the dielectric layer 2 and dried to form a thick film having a laminated structure of three layers, i.e., electrode - dielectric layer - electrode. The thick film is baked in a belt furnace at 800-1000°C for 5-30 minutes under a nitrogen atmosphere to form a thick-film condenser. The dielectric characteristics of the thick-film condenser, i.e., dielectric constant, tan δ (dielectric loss factor), and insulation resistance, and temperature variation percentage of the dielectric constant are measured. The dielectric constant, and tan δ (dielectric loss factor) are measured under electric field of 1 kHz and 1 V/mm. Also, the dielectric resistance is measured one minute after 30 V is applied. Table 5 illustrates the composition of the major components and the sub-components used in the dielectric materials. Also, Table 6 illustrates the dielectric characteristics, i.e., dielectric constant, tan δ (dielectric loss factor), and insulation resistance, and temperature variation percentage of the dielectric constant (a value measured at 20°C is the base value) of the thick-film condenser prepared by baking at 900°C for 10 minutes under a nitrogen atmosphere. As can be seen from Tables 5 and 6, the dielectric materials of this invention (samples Nos. 2-4, 6-11, 15-19, and 23) is sintered sufficiently, and have a lower temperature variation percentage, i.e., a dielectric constant varying within 10% of the value measured at 20°C in the temperature range of -25°C to 85°C, and a high enough insulation resistance, enough for practical uses even though it is baked at a low temperature of 900°C for a short period of time. Especially, the dielectric material has improved temperature characteristics in high temperature areas because of the suitable amount of Zr atoms contained therein reduces its Curie point. On the other hand, control sample No. 5 is not sintered sufficiently at 900°C because of the lower amounts of PbO and CuO, i.e., the absence of PbO or CuO as the sub-component, and therefore provides an unacceptably lower dielectric constant. Although control sample No. 12 containing the excess amounts of PbO and CuO is sintered sufficiently, it has unsatisfactory temperature characteristics of dielectric constant, i.e., a higher temperature variation percentage and increased dielectric loss which is not acceptable for practical uses. As described in Example 1, when a small amount of BTZ is added (control sample No. 1), the dielectric constant of the resulting dielectric material has higher temperature dependence derived from PMN. On the other hand, control sample No. 21 containing a large amount of BTZ is neither sintered sufficiently, nor provides excellent characteristics derived from PMN. Also, control sample No. 20 which contains a large amount of Zr does not provide good temperature characteristics because of a severely reduced Curie point. Moreover, control sample No. 14 is not acceptable because of a low dielectric loss factor derived from the excess amount of Cu contained in the PMN. Example 4 Thick-film condensers are made by the same procedure as in Example 3 except that the powder mixture is prepared by adding the prescribed amounts of Pb₃O₄, PbO₂ and Cu₂O to the dielectric composition represented by the following formula: 0.60[Pb(Mg 1/3 Nb 2/3 )Cu 0.1 O 3.1 ] 0.40Ba(Ti 0.9 Zr 0.1 )O₃ The dielectric characteristics of the thick-film condensers, i.e., dielectric constant tan δ , dielectric loss factor, insulation resistance, and temperature variation percentage of the dielectric constant are also measured in the same manner. Table 7 illustrates the amounts added of Pb₃O₄, PbO₂ and Cu₂O. Table 8 illustrates the dielectric characteristics, i.e., dielectric constant, tan δ (dielectric loss factor), and insulation resistance, and temperature variation percentage of the dielectric constant. The added amount of each Pb₃O₄ and PbO₂ are calculated as that of PbO, and the added amount of Cu₂O as CuO. As can be seen from Tables 7 and 8, the dielectric materials of this invention (samples Nos. 2-4, 6-11, 15-19, and 23) are sintered sufficiently, and have a lower temperature variation percentage, i.e., a dielectric constant varying within 10% of the base value measured at 20°C in the temperature range of -25°C to 85°C, and a high enough insulation resistance for practical uses even though it is baked at a low temperature of 900°C for a short period of time. When Pb₃O₄ or PbO₂ is used as the sub-component in the dielectric composition, better dielectric characteristics are obtained than using PbO, because Pb₃O₄ and PbO₂ are decomposed into PbO and oxygen which aids in removing the resin components at baking temperatures. When Cu₂O is used as the sub-component added in the dielectric composition of major components, almost the same results as using CuO in Example 1 are achieved because Cu₂O is oxidized to CuO by oxygen decomposed from Pb₃O₄ or PbO under atmospheric baking conditions. Additionally, the electrode materials that can be used for the condenser of this invention include those available at the baking temperature of the dielectric material, i.e., 800-1000°C, for example, copper, silver, and silver-palladium alloy, and the like. The use of these electrode materials provides less expensive condensers. Although the dielectric materials are baked under a nitrogen atmosphere in these examples, they may be baked in the atmosphere. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention according to the claims appended hereto.

Claims: A dielectric material consisting essentially of a dielectric porcelain composition in the form of a pre-baked powder, represented by the following general formula, and 1.0-25.0 mole % of lead oxides, the amounts of said lead oxides being calculated as the amount of PbO: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.30, and 0≦b≦0.15. The dielectric material of claim 1, wherein said dielectric porcelain composition is a mixture of Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a and the solid solution of BaTiO₃ and BaZrO₃. The dielectric material of claim 1, wherein said lead oxide is PbO, Pb₃O₄, or PbO₂. The dielectric material of claim 1 which further comprises 1.0-15.0 mole % of copper, the amount of said copper oxides being calculated as the amount of CuO and a being defined as 0≦a≦0.25. The dielectric material of claim 4, comprising a mixture of Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a , and the solid solution of BaTiO₃ and BaZrO₃. The dielectric material of claim 4, wherein said lead oxide is PbO, Pb₃O₄, or PbO₂. The dielectric material of claim 4, wherein said copper oxide is Cu₂O or CuO. A condenser comprising a dielectric layer (2) and electrodes (1, 1′) placed on both sides of said dielectric layer, wherein said dielectric layer is made up essentially of a dielectric material consisting of a dielectric porcelain composition in the form of a pre-baked powder represented by the following general formula and 1.0-25.0 mole % of lead oxides, the amounts of said lead oxides being calculated as the amount of PbO: (1-x)[Pb(Mg 1/3 Nb 2/3 )Cu a O 3+a ] xBa(Ti 1-b Zr b )O₃ wherein x, a and b values satisfy 0.30<x<0.80, 0≦a≦0.30, and 0≦b≦0.15. The condenser of claim 8, wherein said electrode is made from copper, silver, or silver-palladium alloy. A condenser according to claim 8, wherein said dielectric layer further comprises 1.0-15.0 mole % of copper oxides, the amounts of said copper oxides being calculated as the amount of CuO, and a being defined as 0≦a≦0.25. The condenser of claim 10, wherein said electrode is made from copper, silver, or silver-palladium alloy. A condenser according to claim 8 or 10, wherein one lower electrode (3), said dielectric layer (2), and one upper electrode (1) are laminated in this order on a ceramic base plate (4). The condenser of claim 12, wherein said electrode is made from copper, silver, or silver-palladium alloy.

Inventors: GEJIMA SEIJI, HAKOTANI YASUHIKO, KAWAKITA KOJI, KIMURA SUZUSHI, OKINAKA HIDEYUKI, GEJIMA, SEIJI, HAKOTANI, YASUHIKO, KAWAKITA, KOJI, KIMURA, SUZUSHI, OKINAKA, HIDEYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H01G4
IPC codes: H01G4, C04B35, H01B3