Method of producing a monolithic ceramic capacitor

Abstract: A monolithic ceramic capacitor and producing method thereof are disclosed. The ceramic capacitor comprises a plurality of dielectric ceramic layers (2a,2b), a plurality of inner electrodes (1) as formed between the dielectric ceramic layers in such a manner that one end of each inner electrode is exposed out of either end of the dielectric ceramic layers, and outer electrodes (5) as electrically connected with the exposed inner electrodes; in which the inner electrodes (4) each are made of a base metal, and Si oxide layers (8a) or compound layers comprising an Si oxide and at least one component that constitutes said dielectric ceramic layers (2a,2b) and said inner electrodes (4) are formed adjacent to the inner electrodes. The capacitor has excellent thermal impact resistance and wet load resistance characteristics.

Description: FIELD OF THE INVENTION The present invention relates to a producing method of a monolithic ceramic capacitor. .BACKGROUND OF THE INVENTION Monolithic ceramic capacitors comprising conventional dielectric ceramic compositions that consist essentially of BaTiO 3 were problematic in that the compositions are reduced into semiconductors if baked in neutral or reducing atmospheres having a low oxygen partial pressure. Therefore, it was necessary to use, as inner electrodes therein, noble metals, such as palladium, platinum and the like, that do not melt even at temperatures, at which such dielectric ceramic compositions are sintered, and that are not oxidized even when baked in atmospheres having a high oxygen partial pressure, in which such dielectric ceramic compositions can be baked without being converted into semiconductors. However, the noble metals much interfered with the reduction in the costs of the monolithic ceramic capacitors thus produced. In order to solve the above-mentioned problems, it has been desired to use inexpensive base metals such as nickel as the materials of inner electrodes. However, if such base metals are used as the materials of inner electrodes and if they are baked under the conventional conditions, they are oxidized to lose their functions as electrodes. Therefore, in order to successfully use such base metals as inner electrodes, dielectric ceramic compositions having excellent dielectric characteristics are needed that can be baked even in neutral or reducing atmospheres having a low oxygen partial pressure without making the layers of the compositions into semiconductors. As materials that meet such requirements, for example, proposed were compositions of the type of BaTiO 3 -CaZrO 3 -MnO-MgO, such as those disclosed in Japanese Patent Application Laid-Open No. 62-256422; compositions of the type of BaTiO 3 -MnO-MgO-rare earth oxide, such as those in Japanese Patent Application Laid-Open No. 63-103861; compositions of the type of BaTiO 3 -(Mg,Zn,Sr,Ca)O-Li 2 O-SiO 2 -MO (MO: BaO, SrO, CaO), such as those in Japanese Patent Publication No. 61-14610; and compositions of the type of (Ba,Ca,Sr,Mg,Ce)(Ti,Zr)O 3 , such as those in Japanese Patent Application Laid-Open No. 3-263708. However, monolithic ceramic capacitors having inner electrodes of such a base metal were problematic in that, if each dielectric ceramic layer is thinned while increasing the number of the layers to be laminated, some residual stress is produced at the interface between the inner electrode and the dielectric ceramic layer due to the difference in the contraction and also in the thermal expansion between the inner electrodes and the dielectric ceramic layers, thereby worsening the thermal impact resistance of the capacitors. For their reliability in high-temperature and high-humidity conditions, which indicates so-called wet load-resisting characteristics of the capacitors, the capacitors of that type were also problematic in that such their characteristics are worsened for the same reasons as above, if each dielectric ceramic layer is thinned while increasing the number of the layers to be laminated. In order to solve these problems, Japanese Patent Publication No. 7-56850 discloses monolithic ceramic capacitors in which inner electrodes of Ni are bonded to the matrix body via an aluminosilicate layer. However, the disclosed capacitors are still problematic in that their capacitance is low, that their equivalent series resistance is large, and that, if each dielectric ceramic layer is thinned, Al as added to the inner electrode paste diffuses into such thin dielectric ceramic layers, thereby worsening the characteristics of the capacitors. In addition, the thermal impact resistance of the capacitors is low. The aluinosilicate layer is a mixture of oxides of metals such as Ni, Si and Al which originate in the inner electrode paste. Further, firing is carried out under a non-oxidative condition, e.g. 1.5% of H 2 - 98.5% of N 2 . On the other hand, Japanese Patent Application Laid-Open No. 3-133114 discloses monolithic ceramic chip capacitors, in which oxide layers comprising components that are different from those constituting the dielectric layers are formed around the inner electrodes. In this, the samples are fired in atmospheres having a low oxygen concentration for the purpose of improving their reliability in high-temperature load tests. However, this baking method is ineffective in improving the thermal impact resistance and the wet load resistance of the capacitors. In the ceramic chip capacitor mangane oxide layer is formed around an internal electrode. Oxides of Si/Ca/Ba/Ti/Zr/P which originate in the dielectric material may be included in the mangane oxide layer. EP-A-0 623 940 discloses a ceramic multilayer chip capacitor including a first and second plurality of internal electrodes, a plurality of dielectric layers and a pair of external electrodes connected to the first and second internal electrodes, respectively. The material of which the dielectric layers are formed include titanate composite oxides, including BaTiO 3 . The dielectric titanate composite oxides may further contain silicon oxide and/or manganese oxide. SUMMARY OF THE INVENTION The subject matter of the present invention is to improve the dielectric ceramic layers constituting a monolithic ceramic capacitor having inner electrodes of a base metal, thereby providing a monolithic ceramic capacitor having excellent thermal impact resistance and wet load resistance characteristics. The present invention has been made in consideration of the above-mentioned object. The present invention provides a method of producing a monolithic ceramic capacitor which is defined by the features of claim 1. The capacitor may be composed of a plurality of dielectric ceramic layers, a plurality of inner electrodes as formed between the dielectric ceramic layers in such a manner that one end of each inner electrode is exposed out of either end of the dielectric ceramic layers, and outer electrodes as electrically connected with the exposed inner electrodes The inner electrodes each are made of a base metal. Si oxide layers and/or compound layers comprising an Si oxide and at least one component that constitutes said dielectric ceramic layers and said inner electrodes are formed adjacent to the inner electrodes. In the method of the invention, the base metal is preferably Ni or an Ni alloy. In the monolithic ceramic capacitor, at least one component constituting the dielectric ceramic layers is preferably an Si oxide. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view illustrating one embodiment of the monolithic ceramic capacitor made by the method of the present invention. Fig. 2 is a cross-sectional view illustrating another embodiment of the monolithic ceramic capacitor made by the method of the present invention. Fig. 3 is a cross-sectional view illustrating still another embodiment of the monolithic ceramic capacitor made by the method of the present invention. . Fig. 4 is a cross-sectional view illustrating still another embodiment of the monolithic ceramic capacitor made by the method of the present invention. PREFERRED EMBODIMENTS OF THE INVENTION The monolithic ceramic capacitor is composed of a plurality of dielectric ceramic layers, a plurality of inner electrodes as formed between the dielectric ceramic layers in such a manner that one end of each inner electrode is exposed out of either end of the dielectric ceramic layers, and outer electrodes as electrically connected with the exposed inner electrodes, and is characterized in that the inner electrodes each are made of a base metal and that Si oxide layers are formed adjacent to the inner electrodes. As having this constitution, the dielectric ceramic layers in the monolithic ceramic capacitor produced by the method of the invention having inner electrodes of a base metal are improved, and the capacitor has excellent thermal impact resistance and wet load resistance characteristics. Specifically, in the monolithic ceramic capacitor, Si gathers around the inner electrodes during baking and is liable to form a liquid phase therearound, resulting in that the Si oxide layers are liable to spread over the entire surfaces of the electrodes. Accordingly, in this, the bonding between the inner electrodes and the dielectric ceramic layers is enhanced, thereby improving the thermal impact resistance and the wet load resistance characteristics of the capacitor. The Si oxide layers formed adjacent to the inner electrodes in the capacitor may be any of deposits derived from the dielectric ceramic layers and those derived from the inner electrodes. Preferably, the Si oxide layers are deposits derived from the dielectric ceramic layers, as being effective in enlarging the capacitance of the capacitor and in reducing the equivalent series resistance thereof. The composition of the dielectric ceramic layers constituting the capacitor is not specifically defined. For example, usable herein are various dielectric ceramic compositions consisting essentially of BaTiO 3 or SrTiO 3 . For the reasons mentioned above, preferably used are dielectric ceramic compositions comprising Si oxides. The method of the present invention may also provide a monolithic ceramic capacitor which is composed of a plurality of dielectric ceramic layers, a plurality of inner electrodes as formed between the dielectric ceramic layers in such a manner that one end of each inner electrode is exposed out of either end of the dielectric ceramic layers, and outer electrodes as electrically connected with the exposed inner electrodes, and which is characterized in that the inner electrodes each are made of a base metal and that compound layers comprising an Si oxide and at least one component that constitutes said dielectric ceramic layers and said inner electrodes are formed adjacent to the inner electrodes. As having this constitution, the dielectric ceramic layers in the monolithic ceramic capacitor of the invention having inner electrodes of a base metal are improved, and the capacitor has excellent thermal impact resistance and wet load resistance characteristics. Specifically, in the monolithic ceramic capacitor of that type, Si gathers around the inner electrodes during baking and is liable to form a liquid phase therearound, resulting in that the Si-containing oxide layers are liable to spread over the entire surfaces of the electrodes. Accordingly, in this, the bonding between the inner electrodes and the dielectric ceramic layers is enhanced, thereby improving the thermal impact resistance and the wet load resistance characteristics of the capacitor. The compound layers comprising an Si oxide and at least one component that constitutes the dielectric ceramic layers and the inner electrodes, which are formed adjacent to the inner electrodes in the capacitor of the invention, may be any of deposits derived from the dielectric ceramic layers and those derived from the inner electrodes. Preferably, the compound layers are deposits derived from the dielectric ceramic layers, as being effective in enlarging the capacitance of the capacitor and in reducing the equivalent series resistance thereof. The composition of the dielectric ceramic layers constituting the capacitor of that type is not specifically defined. For example, usable herein are various dielectric ceramic compositions consisting essentially of BaTiO 3 or SrTiO 3 . For the reasons mentioned above, preferably used are dielectric ceramic compositions comprising Si oxides. Even base metals are usable to produce the inner electrodes constituting the capacitor. The composition of the base metals for use in the present invention is not specifically defined. For these, for example, concretely referred to are Ni, Ni alloys, Cu and Cu alloys. Preferred are Ni and Ni alloys, since higher sintering temperatures may be applicable to these. The composition of the outer electrodes constituting the capacitor is not limited. For example, the outer electrodes constituting the capacitor may be any of sintered layers of electroconductive powder of Ag, Pd, Ni or the like. That is, usable herein are so-called baked electrodes. The sintered layers for the outer electrodes may comprise such electroconductive powder and glass frit of various types. The glass frit may include, for example, B 2 O 3 -Li 2 O-SiO 2 -BaO, B 2 O 3 -SiO 2 -BaO, Li 2 O-SiO 2 -BaO, and B 2 O 3 -SiO 2 -ZnO. The sintered layers may be coated with a metal plate layer of; for example, Ni, Cu, Ni-Cu or the like, which may be further coated with an additional plate layer of, for example, solder, tin or the like. Now, the present invention is described more concretely hereinunder with reference to Examples, which, however, are not intended to restrict the scope of the invention. Some embodiments of the monolithic ceramic capacitor produced by the method of the present invention are referred to. Fig. 1 to Fig. 4 are cross-sectional views illustrating different embodiments of the monolithic ceramic capacitor. As illustrated, the monolithic ceramic capacitor 1 is a rectangular parallelepiped, chip-type capacitor, which comprises a ceramic laminate 3 as produced by laminating a plurality of dielectric ceramic layers 2a, 2b via an inner electrode 4 therebetween; outer electrodes 5 formed at the both ends of the ceramic laminate 3; a first plate layer 6 of nickel, copper or the like as formed on each outer electrode 5; and a second plate layer 7 of solder, tin or the like as formed on each first plate layer 6. Now, a method for producing the monolithic ceramic capacitor 1 of the invention is described below in the order of the steps constituting the method. First, the ceramic laminate 3 is produced as follows. A non-reducing, dielectric ceramic composition is formed into a slurry, and then sheeted to prepare dielectric ceramic layers 2 (green sheets). On one surface of each green sheet, formed is an internal electrode 4 of nickel or a nickel alloy. To form the internal electrode 4, employable is any method of screen printing, metal vapor deposition or plating. A predetermined number of the dielectric ceramic layers 2b each with the inner electrode 4 formed thereon are laminated, and then sandwiched between two dielectric ceramic layers 2s with no inner electrode 4. Next, the thus-laminated dielectric ceramic layers 2a, 2b, . . . 2b, 2a are baked in a reducing atmosphere at a predetermined temperature to give the ceramic laminate 3. Next, on the both sides of the ceramic laminate 3, formed are two outer electrodes 5 that are connected with the inner electrodes 4. The material of the outer electrodes 5 may be the same as that of the inner electrodes 4. Apart from this, silver, palladium, silver-palladium alloys, copper, copper alloys and others can be used as the material of the outer electrodes 5. If desired, also usable are mixtures comprising powder of such metal material and glass frit of, for example, B 2 O 3 -SiO 2 -BaO, Li 2 O-SiO 2 -BaO or the like. Anyhow, suitable materials are selected for the outer electrodes 5, in consideration of the use of the monolithic ceramic capacitor and the site at which the capacitor is used. The outer electrodes 5 can be formed by applying a paste material of metal powder onto the baked, ceramic laminate 3 followed by baking it. Alternatively, the paste material can be applied onto the non-baked ceramic laminate 3, which is baked all at a time. After this, the outer electrodes 5 may be plated with nickel, copper or the like to form a first plate layer 6 thereon. Last, the first plate layer 6 is coated with a second plate layer 7 of solder, tin or the like. Thus is produced the chip-type, monolithic ceramic capacitor 1. As in Fig. 1 to Fig. 4, (a) Si oxide layers, or (b) compound layers comprising an Si oxide and at least one component constituting the dielectric ceramic layers and inner electrodes are formed adjacent to the inner electrodes 4, and these are in any of the conditions mentioned below. The layers (a) and (b) are hereinafter referred to as Si-containing interlayers 8. (1) The Si-containing interlayers 8 cover almost entirely the both main surfaces of each inner electrode 4, while forming the first layers 8a that face each other via the inner electrode 4 (Fig. 1). (2) The Si-containing interlayers 8 cover partly and randomly the both main surfaces of each inner electrode 4, while forming the first layers 8a that face each other via the inner electrode 4 (Fig. 2). (3) The Si-containing interlayers 8 cover almost entirely the both main surfaces of each inner electrode 4, while forming different first layers 8a and second layers 8b that face each other via the inner electrode 4 (Fig. 3). (4) The Si-containing interlayers 8 cover partly and randomly the both main surfaces of each inner electrode 4, while forming the first layers 8a that face each other via the inner electrode 4 and forming the second layers 8b that cover the first layers 8a and face each other via the inner electrode 4 (Fig. 4). Now, the constitution of the monolithic ceramic capacitor is described in more detail hereinunder with reference to the following Examples. EXAMPLE 1 First, raw materials of TiCl 4 and Ba(NO 3 ) 2 having various degrees of purity were prepared and weighed. These were treated with oxalic acid to obtain a precipitate of barium titanyl oxalate (BaTiO(C 2 O 4 )4H 2 O). This precipitate was decomposed under heat at 1000 i C or higher to give barium titanate (BaTiO 3 ) having a molar ratio of Ba/Ti of 1.0. Next prepared were BaCO 3 , which is to vary the molar ratio of Ba/Ti of this barium titanate, and also Y 2 O 3 , Dy 2 O 3 , MnCO 3 , NiO, Co 2 O 3 and MgO each having a purity of 99 % or higher. The thus-prepared, powdery materials were formulated to give a composition consisting essentially of a main component of 97.0{BaO} 1.010 TiO 2 + 0.7Y 2 O 3 + 0.3Dy 2 O 3 + 0.6MnO + 0.7NiO + 0.7CoO (as mol%) and containing 1.2 mol%, relative to said main component of being 100 mol%, of MgO, to which was added 1.5 wt.%, relative to the sum of said main component and MgO of being 100 wt.%, of oxide glass consisting essentially of Li 2 O-(TiO 2 SiO 2 )-Al 2 O 3 . Next, a polyvinyl butyral binder and an organic solvent, such as ethanol or the like, were added thereto, and wet-milled in a ball mill to prepare a ceramic slurry. The resulting ceramic slurry was sheeted according to doctor blading to obtain a rectangular, green sheet having a thickness of 11 µm. Next, an electroconductive paste consisting essentially of Ni was printed on this ceramic green sheet, according to screen printing, to form thereon an electroconductive paste layer, which is formed into an inner electrode. A plurality of these ceramic green sheets each having the electroconductive paste layer formed thereon were laminated in such a manner that the side of one sheet with the electroconductive paste exposed out of it was alternated with that of another sheet with the electroconductive paste not exposed out of it. Thus was obtained a laminate. This laminate was heated in an N 2 atmosphere at 350 i C whereby the binder was burnt out, and then baked in a reducing atmosphere comprising gases of H 2 , N 2 and H 2 O under the condition indicated in Table 1. Thus were obtained herein various ceramic laminates as shown in Table 1. To bake them, the samples were kept heated at 1300°C for 2 hours, and the heating speed and the cooling speed were both 200°C/hr. A silver paste was applied onto the both sides of each sintered ceramic laminate, and again baked in an N 2 atmosphere at 600°C to thereby form outer electrodes as electrically connected with the inner electrodes. Next, the outer electrodes were coated with an Ni plate film, which was further coated with a solder plate film. The outer dimension of each of these monolithic ceramic capacitors thus obtained was 1.6 mm width x 3.2 mm length x 1.2 mm thickness, and the thickness of each dielectric ceramic layer as sandwiched between the inner electrodes was 6 µm. The total number of the effective dielectric ceramic layers was 150. To determine the thermal impact resistance of the capacitors produced herein, 50 pieces of each sample were subjected to a thermal impact test, in which the test pieces each as pinched in a pair of tweezers were dipped in a solder tank kept at 300°C or 325°C for a few seconds. After having been hardened with a resin, the samples were polished and then observed with a microscope to check the presence or absence of cracks formed. To determine the wet load resistance of the capacitors, 72 pieces of each sample were subjected to a wet load test, in which a direct current voltage of 16 V was applied to each test piece under a pressure of 2 atmospheres, at a relative humidity of 100 % and at a temperature of 121°C, while measuring the insulating resistance of each test piece which varied time-dependently. In this test, the number of the tested pieces which showed an insulating resistance (R) of 10 6 □ or lower within a period of 250 hours and which therefore failed in this test was counted. To confirm the presence of the Si-containing interlayers existing adjacent to the inner electrodes, each sample was subjected to EPMA analysis. The results obtained in these tests are shown in Table 1. Sample No. Oxygen Pressure at 1300°C (MPa) Si-Containing Interlayers Thermal Impact Test Wet Load Test 300°C 325°C 1-1 10 -12 No 3/50 7/50 3/72 1-2 10 -11 No 1/50 4/50 2/72 1-3 10 -10 Yes 0/50 0/50 0/72 1-4 10 -8 Yes 0/50 0/50 0/72 1-5 10 -6 Yes As cracked, the sample could not be tested. In Sample No. 1-1 and Sample No. 1-2 which had been sintered in the atmosphere having an oxygen concentration of 10 -11 MPa or lower, Si-containing interlayers were not formed; while in Sample Nos. 1-3, 1-4 and 1-5 which had been sintered in the atmosphere having an oxygen concentration of 10 -10 MPa or higher, formed were interlayers comprising oxides of Si, rare earth elements, Mn, Ni, Co and Mg around the inner electrodes, as in Figs. 1 to 4. The interlayers were formed especially greatly around the inner electrodes nearest to the both main surfaces of the laminate and around the edges of the other inner electrodes. However, Sample No. 1-5 was cracked owing to the peroxidation of the inner electrodes, and was not worthy of being evaluated as a capacitor. Referring to the data in the thermal resistance test and the wet load test, some of the test pieces of Sample Nos. 1-1 and 1-2, in which no Si-containing interlayers had been formed, failed in these tests; while none of the test pieces of Sample Nos. 1-3 and 1-4 having Si-containing interlayers failed. These data verify that the Si-containing interlayers formed in the capacitor samples are effective in improving the reliability of the samples in the thermal impact test and the wet load test. The Si-containing interlayers were formed when the ceramic laminates were baked in the atmosphere having a relatively high oxygen concentration. For forming such Si-containing interlayers, it is effective to bake ceramic laminates in atmospheres having a higher oxygen concentration at higher temperatures. The Si-containing interlayers are not limited to only those formed through baking, but could be formed through post heat-treatment after baking with no problem. EXAMPLE 2 First, raw materials of TiCl 4 and Ba(NO 3 ) 2 having various degrees of purity were prepared and weighed. These were treated with oxalic acid to obtain a precipitate of barium titanyl oxalate (BaTiO(C 2 O 4 )4H 2 O). This precipitate was decomposed under heat at 1000 i C or higher to give barium titanate (BaTiO 3 ) having a molar ratio of Ba/Ti of 1.0. Next prepared were Ho 2 O 3 , Co 2 O 3 , BaCO 3 , MnCO 3 , MgO and SiO 2 each having a purity of 99 % or higher. The thus-prepared, powdery materials were formulated to give a composition consisting essentially of a main component of 96.5BaTiO 3 + 1.5Ho 2 O 3 + 2.0Co 2 O 3 (as mol%) and containing 0.5 mol% of BaO, 1.5 mol% of MnO, 2.0 mol% of MgO and 2.0 mol% of SiO 2 , all relative to said main component of being 100 mol%. Next, a polyvinyl butyral binder and an organic solvent, such as ethanol or the like, were added thereto, and wet-milled in a ball mill to prepare a ceramic slurry. The resulting ceramic slurry was sheeted according to doctor blading to obtain a rectangular, green sheet having a thickness of 11 µm. Next, an electroconductive paste consisting essentially of Ni was printed on this ceramic green sheet, according to screen printing, to form thereon an electroconductive paste layer, which is formed into an inner electrode. A plurality of these ceramic green sheets each having the electroconductive paste layer formed thereon were laminated in such a manner that the side of one sheet with the electroconductive paste exposed out of it was alternated with that of another sheet with the electroconductive paste not exposed out of it. Thus was obtained a laminate. This laminate was heated in an N 2 atmosphere at 350°C whereby the binder was burnt out, and then baked in a reducing atmosphere comprising gases of H 2 , N 2 and H 2 O under the condition indicated in Table 2. Thus were obtained herein various ceramic laminates as shown in Table 2. To bake them, the samples were kept heated at 1300°C for 2 hours, and the heating speed and the cooling speed were both 200 i C/hr. A silver paste was applied onto the both sides of each sintered ceramic laminate, and again baked in an N 2 atmosphere at 600°C to thereby form outer electrodes as electrically connected with the inner electrodes. Next, the outer electrodes were coated with an Ni plate film, which was further coated with a solder plate film. The outer dimension of each of these monolithic ceramic capacitors thus obtained was 1.6 mm width x 3.2 mm length x 1.2 mm thickness, and the thickness of each dielectric ceramic layer as sandwiched between the inner electrodes was 6 µm. The total number of the effective dielectric ceramic layers was 150. To determine the thermal impact resistance of the capacitors produced herein, 50 pieces of each sample were subjected to a thermal impact test, in which the test pieces each as pinched in a pair of tweezers were dipped in a solder tank kept at 300°C or 325°C for a few seconds. After having been hardened with a resin, the samples were polished and then observed with a microscope to check the presence or absence of cracks formed. To determine the wet load resistance of the capacitors, 72 pieces of each sample were subjected to a wet load test, in which a direct current voltage of 16 V was applied to each test piece under a pressure of 2 atmospheres, at a relative humidity of 100 % and at a temperature of 121°C, while measuring the insulating resistance of each test piece which varied time-dependently. In this test, the number of the tested pieces which showed an insulating resistance (R) of 10 6 □ or lower within a period of 250 hours and which therefore failed in this test was counted. To confirm the presence of the Si-containing interlayers existing adjacent to the inner electrodes, each sample was subjected to EPMA analysis. The results obtained in these tests are shown in Table 2. Sample No. Oxygen Pressure at 1300°C (MPa) Si-Containing Interlayers Thermal Impact Test Wet Load Test 300°C 325°C 2-1 10 -12 No 4/50 6/50 5/72 2-2 10 -11 No 1/50 3/50 3/72 2-3 10 -10 Yes 0/50 0/50 0/72 2-4 10 -8 Yes 0/50 0/50 0/72 2-5 10 -8 Yes As cracked, the sample could not be tested. In Sample No. 2-1 and Sample No. 2-2 which had been sintered in the atmosphere having an oxygen concentration of 10 -11 MPa or lower, Si-containing interlayers were not formed; while in Sample Nos. 2-3, 2-4 and 2-5 which had been sintered in the atmosphere having an oxygen concentration of 10 -10 MPa or higher, formed were interlayers comprising oxides of Si, rare earth elements, Mn, Ni, Co and Mg, as in Figs. 1 to 4. The interlayers were formed especially greatly around the inner electrodes nearest to the both main surfaces of the laminate and around the edges of the other inner electrodes. However, Sample No. 2-5 was cracked owing to the peroxidation of the inner electrodes, and was not worthy of being evaluated as a capacitor. Referring to the data in the thermal resistance test and the wet load test, some of the test pieces of Sample Nos. 2-1 and 2-2, in which no Si-containing interlayers had been formed, failed in these tests; while none of the test pieces of Sample Nos. 2-3 and 2-4 having Si-containing interlayers failed. These data verify that the Si-containing interlayers formed in the capacitor samples are effective in improving the reliability of the samples in the thermal impact test and the wet load test. The Si-containing interlayers were formed when the ceramic laminates were baked in the atmosphere having a relatively high oxygen concentration. For forming such Si-containing interlayers, it is effective to bake ceramic laminates in atmospheres having a higher oxygen concentration at higher temperatures. The Si-containing interlayers are not limited to only those formed through baking, but could be formed through post heat-treatment after baking with no problem. In accordance with the examples 1 and 2, acceptable oxygen pressure ranges from 10 -10 to 10 -8 MPa at 1300°C. The equilibrium oxgen pertial pressure of Ni/NiO is 10 -7.3 MPa at 1300°C. From this, it is preferable that firing is carried out an oxygen pressure near or under the equilibrium oxygen partial pressure of M/MO (M is a metal forming an internal electrode). And, the lower limit of the oxgen partial pressure 1/1000 of the equilibrium oxygen partial pressure of M/MO. EXAMPLE 3 Two different samples were prepared herein. One was comprised of the same main component of the dielectric ceramic composition as in Example 1 and 1.0 wt.%, relative to 100 wt.% of the dielectric ceramic composition, of the same oxide glass containing SiO 2 as in Example 1; while the other was comprised of only the main component of the dielectric ceramic composition but not contained the oxide glass. Using these, monolithic ceramic capacitors were formed in the same manner as in Example 1. The oxygen concentration in the baking atmosphere used herein was the same as that for the baking of Sample No. 1-4 shown in Table 1. The baking temperature for the former sample containing the oxide glass was 1300°C; while that for the latter sample not containing the oxide glass was 1380°C. The higher temperature for the latter was to compensate the sintering insufficiency. To determine the thermal impact resistance of the capacitors produced herein, 50 pieces of each sample were subjected to a thermal impact test, in which the test pieces each as pinched in a pair of tweezers were dipped in a solder tank kept at 300°C or 325°C for a few seconds. After having been hardened with a resin, the samples were polished and then observed with a microscope to check the presence or absence of cracks formed. To determine the wet load resistance of the capacitors, 72 pieces of each sample were subjected to a wet load test, in which a direct current voltage of 16 V was applied to each test piece under a pressure of 2 atmospheres, at a relative humidity of 100 % and at a temperature of 121°C, while measuring the insulating resistance of each test piece which varied time-dependently. In this test, the number of the tested pieces which showed an insulating resistance (R) of 10 6 □ or lower within a period of 250 hours and which therefore failed in this test was counted. To confirm the presence of the Si-containing interlayers existing adjacent to the inner electrodes, each sample was subjected to EPMA analysis. The results obtained in these tests are shown in Table 3. Sample No. Glass Component Si-Containing Interlayers Thermal Impact Test Wet Load Test 300°C 325°C 3-1 1.0 wt.% added. Yes 0/50 0/50 0/72 3-2 Not added. No 1/50 12/50 8/72 In Sample No. 3-1 containing the oxide glass, formed were interlayers comprising oxides of Si, rare earth elements, Mn, Ni, Co and Mg, as in Figs. 1 to 4; while in Sample No. 3-2 not containing the oxide glass, no Si-containing layers were formed. Referring to the data in the thermal resistance test and the wet load test, none of the test pieces of Sample No. 3-1 having Si-containing interlayers failed in these tests; while some of the test pieces of Sample No. 3-2, in which no Si-containing interlayers had been formed, failed in both the thermal resistance test and the wet load test. These data verify that the Si-containing interlayers formed in the sample are effective in improving the reliability of the sample in the thermal impact test and the wet load test. The Si-containing interlayers were formed when the ceramic laminate was baked in the atmosphere having a relatively high oxygen concentration. For forming such Si-containing interlayers, it is effective to bake ceramic laminates in atmospheres having a higher oxygen concentration at higher temperatures. The Si-containing interlayers are not limited to only those formed through baking, but could be formed through post heat-treatment after baking with no problem. As having Si-containing interlayers around the interfaces between the inner electrodes of a base metal and the dielectric ceramic layers, the monolithic ceramic capacitor produced by the method of the present invention has better thermal impact resistance and wet load resistance characteristics than any other conventional ones. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the scope thereof as defined by the appended claims.

Claims: A method of producing a monolithic ceramic capacitor (1) comprising the steps of: providing a ceramic green sheet including barium titanate as a main component, magnesium oxide as a sub component, and oxide glass including Li 2 O-(TiO 2 •SiO 2 )-Al 2 O 3 as a main component; depositing an inner electrode (4) made of a base metal M on said green sheet, said metal M being capable of forming an oxide MO; laminating at last two of said green sheets; firing said laminated body in an atmosphere having an oxygen concentration which corresponds to an oxygen partial pressure which ranges from 10 -a-3 Mpa to 10 -a Mpa, where 10 -a Mpa is the equilibrium oxygen partial pressure of M/MO; and depositing an external electrode (5) which is electrically connected to said inner electrode, on the outer surface of said laminated body. A method of producing a monolithic ceramic capacitor according to claim 1, wherein said base metal is nickel.

Inventors: MIYAZAKI TAKAHARU, SAKAMOTO NORIHIKO, SANO HARUNOBU, MIYAZAKI, TAKAHARU, SAKAMOTO, NORIHIKO, SANO, HARUNOBU
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1997
Main classification: H01G4
IPC codes: H01G4, H01B3, C04B35