Dielectric ceramic composition

Abstract: Disclosed is a dielectric ceramic composition having a high dielectric constant, which can be fired along with silver and silver alloys at temperatures not higher than 1000°C. To a two-component dielectric ceramic composition comprising a solid solution that consists essentially of lead nickeloniobate Pb(Ni 1/3 Nb 2/3 )O 3 and lead titanate PbTiO 3 , added is at least one additive selected from the group consisting of a group of compounds of Pb 3 O 4 , CuO and WO 3 capable of forming a composition of a chemical formula Pb(Cu 1/2 W 1/2 )O 3 , and manganese oxide. The additives have the ability to promote the sintering of the two-component composition and to lower the firing temperature for the composition.

Description: FIELD OF THE INVENTION The present invention relates to a ceramic composition, in particular, to a dielectric ceramic composition capable of being fired at relatively low temperatures. BACKGROUND OF THE INVENTION With the recent demand for small-sized, large-capacity devices in ceramic capacitors, monolithic ceramic capacitors are being increasingly popularized. Monolithic ceramic capacitors are produced by alternately laminating inner electrode layers and dielectric ceramic layers followed by firing the resulting laminate of those inner electrode layers and dielectric ceramic layers, for which are used highly dielectric, ceramic capacitor materials having a high dielectric constant. The monolithic ceramic capacitors thus produced shall have a large capacitance. Heretofore, barium titanate-type materials have been widely used as such highly dielectric, ceramic capacitor materials. However, those materials must be fired at high temperatures of not lower than 1300°C. Therefore, where they are used in producing monolithic ceramic capacitors, expensive noble metals such as platinum and palladium must be used to form the inner electrodes in the capacitors. On the other hand, some recent reports have proposed the use of lead-containing composite perovskite compounds, which can be fired at low temperatures and have a high dielectric constant, as the materials for monolithic ceramic capacitors. For example, it is reported in JP-B-01-46471 that a two-component composition comprising lead nickeloniobate Pb(Ni 1/3 Nb 2/3 )O 3 and lead titanate PbTiO 3 has an extremely high dielectric constant. However, the firing temperature for the conventional barium titanate-type materials is not lower than 1300°C and that for the conventional lead-containing composite perovskite compounds is about 1100°C. Therefore, all those conventional materials are problematic in that they could not be fired along with inexpensive silver and silver alloys. In particular, where the conventional barium titanate-type materials and lead-containing composite perovskite compounds are used to produce monolithic ceramic capacitors, inexpensive silver or silver alloys could not be employed to form the inner electrodes for those capacitors, resulting in that it is impossible to produce monolithic ceramic capacitors at low production costs. SUMMARY OF THE INVENTION The object of the present invention is to solve the problems mentioned above and to provide a dielectric ceramic composition having a high dielectric constant and capable of being fired at a temperature of 1000°C or lower along with silver and silver alloys. This object is achieved with a dielectric ceramic composition having the features of claim 1. The subclaims are directed to preferable embodiments. After having studied the firing condition for the two-component composition comprising lead nickeloniobate Pb(Ni 1/3 Nb 2/3 )O 3 and lead titanate PbTiO 3 and also various additives thereto capable of promoting the sintering of the composition, the present inventors have found that manganese oxide (MnO), a group of compounds of Pb 3 O 4 , CuO and WO 3 capable of forming a composition of Pb(Cu 1/2 W 1/2 )O 3 can promote the sintering of the two-component composition while lowering the firing temperature for the composition, and have completed the present invention. Preferably, the solid solution is represented by a formula, XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 , in which X indicates a molar fraction to fall within the scope of 0.57 ≤ X ≤ 0.87, in order to make the composition have a relatively high specific inductive capacity. More preferably, the solid solution is represented by a formula, XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 , in which X indicates a molar fraction to fall within the scope of 0.65 ≤ X ≤ 0.75, in order to make the composition have a higher specific inductive capacity. Preferably, the amount is manganese oxide (MnO) to be in the dielectric ceramic composition is not larger than 1 % by weight relative to the solid solution, in order not to lower the specific inductive capacity of the composition. Also preferably, the amount of the group of compounds of Pb 3 O 4 , CuO and WO 3 to be in the dielectric ceramic composition is not larger than 9 % by weight in terms of Pb(Cu 1/2 W 1/2 )O 3 and relative to the essential component of the composition, in order not to lower the specific inductive capacity of the composition. More preferably, the dielectric ceramic composition of the invention contains both the group of compounds of Pb 3 O 4 , CuO and WO 3 capable of forming Pb(Cu 1/2 W 1/2 )O 3 , and manganese oxide, in order to further lower the firing temperature for the composition, in which the amount of manganese oxide (MnO) is not larger than 1 % by weight relative to the essential component of the composition and that of the group of compounds of Pb 3 O 4 , CuO and WO 3 is not larger than 9 % by weight in terms of Pb(Cu 1/2 W 1/2 )O 3 and relative to the solid solution, in order not to lower the specific inductive capacity of the composition. Brief Description of the Drawing Fig. 1 shows the outline of the flow chart for one embodiment of producing the dielectric ceramic composition of the invention. Fig. 2 shows the sectional view of a monolithic ceramic capacitor in accordance with the present invention. Description of Embodiments of the Invention Now, embodiments of the dielectric ceramic composition of the invention are described hereinunder. Embodiment 1: Embodiment 1 of the invention is the dielectric ceramic composition which consists essentially of a solid solution of a formula XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 , where X indicates a molar fraction, and which contains manganese oxide MnO that acts to lower the firing temperature for the solid solution. In Embodiment 1, the amount of manganese oxide to be added to the composition is preferably not larger than 1 % by weight relative to the essential component, in order not to lower the high specific inductive capacity, ε, of the essential component, solid solution. The dielectric ceramic composition of Embodiment 1 having the constitution mentioned above can be fired at low temperatures of 1000°C or lower, while having a relatively high specific inductive capacity, ε. Embodiment 2: Embodiment 2 of the invention is the dielectric ceramic composition which consists essentially of a solid solution of a formula XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 , where X indicates a molar fraction, and which contains a group of compounds Pb 3 O 4 , CuO and WO 3 that acts to lower the firing temperature for the solid solution. In this, the group of compounds Pb 3 O 4 , CuO and WO 3 can form a composition of a chemical formula Pb(Cu 1/2 W 1/2 )O 3 . In Embodiment 2, the amount of the group of compounds Pb 3 O 4 , CuO and WO 3 to be added to the dielectric ceramic composition is preferably not larger than 9 % by weight in terms of Pb(Cu 1/2 W 1/2 )O 3 and relative to the essential component, in order not to lower the high specific inductive capacity, ε, of the essential component, solid solution. The dielectric ceramic composition of Embodiment 2 having the constitution mentioned above can be fired at low temperatures of 1000°C or lower, while having a relatively high specific inductive capacity, ε. Embodiment 3: Embodiment 3 of the invention is the dielectric ceramic composition which consists essentially of a solid solution of a formula XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 , where X indicates a molar fraction, and which contains both manganese oxide MnO and a group of compounds Pb 3 O 4 , CuO and WO 3 , both acting to lower the firing temperature for the solid solution. In this, the group of compounds Pb 3 O 4 , CuO and WO 3 can form a composition of a chemical formula Pb(Cu 1/2 W 1/2 )O 3 . In Embodiment 3, the amount of manganese oxide MnO to be added to the dielectric ceramic composition is preferably not larger than 1 % by weight relative to the essential component and that of the group of compounds Pb 3 O 4 , CuO and WO 3 to be added thereto is preferably not larger than 9 % by weight in terms of Pb(Cu 1/2 W 1/2 )O 3 and relative to the essential component, in order not to lower the high specific inductive capacity, ε, of the essential component, solid solution. The dielectric ceramic composition of Embodiment 3 having the constitution mentioned above can be fired at lower temperatures than that of Embodiment 1 and Embodiment 2, while having a relatively high specific inductive capacity, ε. In those Embodiments 1 to 3, the solid solution of the essential component of a formula XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 , where X indicates a molar fraction, is preferably so defined that X in that formula falls within the range of 0.57 ≤ X ≤ 0.87 in order to make the dielectric ceramic composition have a high specific inductive capacity, ε, but is more preferably so defined that X therein falls within the range of 0.65 ≤ X ≤ 0.75 in order to make the composition have a higher specific inductive capacity, ε. Now, referring to Fig. 1, hereinunder described is a method for producing the dielectric ceramic composition of embodiments 1 to 3. The method for producing the dielectric ceramic is not part of the claimed invention. In this method Pb 3 O 4 , NiO, Nb 2 O 5 , TiO 2 , MnO, CuO and WO 3 are weighed to give the essential component and the additive in a predetermined ratio in the step S1. In the step S2, the thusweighed materials are mixed in a wet or dry system. Then, in the step S3, the resulting mixture is pre-fired at a predetermined temperature to give a solid solution of a dielectric ceramic composition material. Next, in the step S4, this dielectric ceramic composition material is powdered to obtain a powder of the material. In the next step S5, the powder is shaped to have a predetermined shape along with a resin binder or the like. Then, in the step S6, this is fired at a relatively low, predetermined temperature of not higher than 1000°C, whereby the powdery composition is sintered into a dielectric ceramic composition. For example, where the dielectric ceramic composition of the invention is used to produce a monolithic ceramic capacitor 1 whose sectional view is shown in Fig. 2, a resin binder such as polyvinyl alcohol is added to the powdery dielectric ceramic composition and the resulting mixture is shaped into green sheets in the step S5. Then, those green sheets are alternately laminated with electrode layers made of silver 4 or a silver alloy, the resulting laminate is cut to have a predetermined shape, an electrode paste 6 is applied to the edges of the thus-shaped laminate to form edge electrodes 5 therearound, and finally the laminate thus composed of the green sheets and the electrodes is fired all at once to obtain a monolithic ceramic capacitor. If necessary, a first 6 and a second plating layers 7 may be applied on the electrodes 7. In the monolithic ceramic capacitors comprising the dielectric ceramic composition of any of those embodiments 1 to 3, the composition can be fired at low temperatures of not higher than 1000°C. Therefore, any of inexpensive silver and silver alloys can be used to form the inner electrodes of those capacitors, in place of expensive noble metals such as platinum and palladium, and the capacitors can be produced at low costs. Now, examples of the invention are described below with reference to Table 1. Examples: In these examples, starting materials of Pb 3 O 4 , NiO, Nb 2 O 5 , TiO 2 , MnO, CuO and WO 3 were wet-milled in a ball mill to give final compositions as in Table 1, then vaporized and dried. The compositions in Table 1 are fired ones. Next, each powdery mixture was pre-fired at 750°C for 2 hours to obtain a predetermined, powdery dielectric ceramic composition. Next, this was wet-milled along with 5 parts by weight of a vinyl acetate-type binder in a ball mill. Then, the wet mixture was vaporized, dried and dressed, and the resulting powdery mixture was shaped under a pressure of 2.5 tons/cm 2 to give discs each having a diameter of 10 mm and a thickness of 1.2 mm. These discs were fired in an electric furnace having a Pb atmosphere, at different temperatures shown in Table 1. Next, an Ag paste to be an electrode was applied to each disc, and fired at 800°C to prepare samples. The specific inductive capacity, ε, and the dielectric loss (tan δ) of each sample were measured at 1 KHz, 1 Vrms and 20°C. To measure its specific resistance, ρ, a voltage of 250 V was applied to each sample at 25°C for 120 seconds. To prepare the samples of the dielectric ceramic composition, the molar fraction X in the chemical formula XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 to represent the essential component constituting the composition was varied within the defined range, as in Table 1, while the amount, α % by weight relative to the essential component, of the additive of manganese oxide (MnO) added to the essential component as well as the amount, β % by weight in terms of Pb(Cu 1/2 W 1/2 )O 3 and relative to the essential component, of the additive of the group of Pb 3 O 4 , CuO and WO 3 added thereto was varied within the defined range, as in Table 1. The firing temperatures for those samples, and also the specific inductive capacity ε, the dielectric loss tan δ, and/the specific resistivity ρ of each sample are shown in Table 1. XPb(Ni 1/3 Nb 2/3 )O 3 +(1-X)PbTiO 3 +αMnO+βPb(Cu 1/2 W 1/2 )O 3 Sample No. X of Pb(Ni 1/3 Nb 2/3 )O 3 (1-X) of PbiO 3 (molar fraction) Amount α, of MnO Added (wt.%) Amount, β, of Pb(Cu 1/2 W 1/2 )O 3 Firing Temperature (°C) ε tan δ (%) ρ (Ωcm) * 1 0.87 0.13 0 0 1100 2940 0.5 7.52x10 10 * 2 0.80 0.20 0 0 1100 6480 0.8 6.92x10 10 * 3 0.70 0.30 0 0 1100 14200 1.6 2.88x10 10 * 4 0.60 0.40 0 0 1100 4480 2.9 8.12x10 9 * 5 0.57 0.43 0 0 1100 2870 4.6 3.47x10 9 6 0.87 0.13 0.5 0 990 2060 0.3 2.57x10 12 7 0.80 0.20 0.5 0 990 4540 0.4 1.02x10 12 8 0.75 0.25 0.5 0 990 5340 0.5 8.73x10 11 9 0.70 0.30 0.5 0 990 10800 0.9 7.54x10 11 10 0.65 0.35 0.5 0 990 5570 1.2 3.31x10 11 11 0.60 0.40 0.5 0 990 3450 1.7 1.74x10 11 12 0.57 0.43 0.5 0 990 2010 2.6 1.12x10 11 13 0.87 0.13 1 0 970 1760 0.4 2.42x10 12 14 0.80 0.20 1 0 970 3940 0.4 9.94x10 11 15 0.75 0.25 1 0 970 5250 0.5 8.77x10 11 16 0.70 0.30 1 0 970 8550 0.8 7.12x10 11 17 0.65 0.35 1 0 970 5060 1.0 3.96x10 11 18 0.60 0.40 1 0 970 2690 1.5 2.32x10 11 19 0.57 0.43 1 0 970 1720 2.1 1.89x10 11 20 0.87 0.13 0 3 960 2740 0.4 7.61x10 11 21 0.80 0.20 0 3 960 4830 0.9 6.72x10 11 22 0.75 0.25 0 3 960 5780 1.3 5.68x10 11 23 0.70 0.30 0 3 960 12790 1.5 3.93x10 11 24 0.65 0.35 0 3 960 5110 2.1 3.19x10 11 25 0.60 0.40 0 3 960 4040 3.1 2.55x10 11 26 0.57 0.43 0 3 960 2580 4.5 1.56x10 11 27 0.87 0.13 0 9 900 1860 1.0 6.83x10 11 28 0.80 0.20 0 9 900 4080 1.3 6.21x10 11 29 0.75 0.25 0 9 900 5490 1.7 4.93x10 11 30 0.70 0.30 0 9 900 8940 2.1 4.15x10 11 31 0.65 0.35 0 9 900 5210 2.9 4.08x10 11 32 0.60 0.40 0 9 900 2830 3.5 3.86x10 11 33 0.57 0.43 0 9 900 1800 5.1 2.03x10 11 34 0.87 0.13 0.5 3 950 1950 0.3 3.61x10 12 35 0.80 0.20 0.5 3 950 4310 0.4 1.36x10 12 36 0.70 0.30 0.5 3 950 10280 1.0 8.71x10 11 37 0.60 0.40 0.5 3 950 3280 1.9 2.34x10 11 38 0.57 0.43 0.5 3 950 1910 3.1 9.67x10 11 Samples with * are outside the scope of the invention. As is obvious from Table 1, the samples of Nos. 1, 2, 3, 4 and 5, which contained none of the additive of manganese oxide (MnO) and the additive of the group of Pb 3 O 4 , CuO and WO 3 and which are outside the scope of the invention, all had a low specific resistivity, ρ < 10 11 Ω ·cm, or that is, those samples all had relatively low insulation resistance, and, in addition, those samples required a high firing temperature of 1100°C. As opposed to these, it is known that the samples of Nos. 6 to 30 which are within the scope of the invention all had a high specific resistivity, ρ > 10 11 Ω·cm, or that is, those samples all had high insulation resistance, and that those samples were successively fired at temperatures lower than 1000°C. As is also obvious from Table 1, the samples that satisfy the requirement for 0.65 ≤ X ≤ 0.75 had a much higher specific inductive capacity ε. In particular, the samples of Nos. 9, 16, 23, 30 and 36 with X = 0.7 had a highest specific inductive capacity ε relative to the varying X in the other samples. Though not shown in Table 1, samples with X < 0.57 and those with 0.87 < X could be fired at temperatures lower than 1000°C, but their specific inductive capacity ε was low to be not higher than 1700. Therefore, in the present invention, preferred is 0.57 ≤ X ≤ 0.87, more preferred is 0.65 ≤ X ≤ 0.75, and most preferred is X = 0.7. Though not shown in Table 1, where 1 < α or where 9 < β, the specific inductive capacity ε of the dielectric ceramic composition of the invention is often significantly lowered. Therefore, in the invention, preferred is α ≤ 1 or β ≤ 9 (provided that α + β ≠ 0). As has been described hereinabove, the dielectric ceramic composition of the invention can be fired at temperatures lower than 1000°C, while having a high specific inductive capacity ε and high insulation resistance. Therefore, when the dielectric ceramic composition of the invention is used, it is possible to provide monolithic ceramic capacitors having large capacitance and high insulation resistance and, in addition, inexpensive silver and silver alloys can be used to form the inner electrodes in those monolithic ceramic capacitors. Accordingly, using the composition of the invention, good monolithic ceramic capacitors can be produced at low costs.

Claims: A dielectric ceramic comprising: a solid solution including lead nickeloniobate Pb(Ni 1/3 Nb 2/3 )O 3 and lead titanate PbTiO 3 , and a composition represented by a formula Pb(Cu 1/2 W 1/2 )O 3 . A dielectric ceramic according to claim 1, further comprising: a manganese oxide. A dielectric ceramic composition according to any one of claims 1 or 2, wherein said solid solution is represented by a formula: XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 wherein X represents a molar fraction to fall within the scope of 0.57 ≤ X ≤ 0.87. A dielectric ceramic according to any one of claims 1 or 2, wherein said solid solution is represented by a formula: XPb(Ni 1/3 Nb 2/3 )O 3 -(1-X)PbTiO 3 . wherein X represents a molar fraction to fall within the scope of 0.65 ≤ X ≤ 0.75. A dielectric ceramic composition according to claim 2, which contains said manganese oxide in an amount of not larger than 1 % by weight relative to the solid solution. A dielectric ceramic composition according to claim 1 or 2, which contains Pb(Cu ½ W ½ )O 3 in an amount of not larger than 9 % by weight relative to the solid solution.

Inventors: MATOBA HIROAKI, MATOBA, HIROAKI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: C04B35
IPC codes: C04B35, H01G4, H01B3