Monolithic ceramic capacitor - Patent Review 5731950

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FIELD OF THE INVENTION

The present invention relates to monolithic ceramic capacitors to be used in electronic equipment, especially those having inner electrodes made of nickel or nickel alloys.

BACKGROUND OF THE INVENTION

Monolithic ceramic capacitors comprising conventional dielectric materials that are constituted essentially of BaTiO.sub.3 were problematic in that the materials 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 that do not melt even at temperatures at which such dielectric ceramic materials are sintered, and that are not oxidized even when baked in atmospheres having a high oxygen partial pressure in which such dielectric ceramic materials can be baked and converted into semiconductors. Accordingly, noble metals such as palladium and platinum must be used as the inner electrodes of monolithic ceramic capacitors, but this interferes with any attempt to reduce the cost 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 material for the 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 and lose their function as electrodes. Therefore, in order to successfully use such base metals as inner electrodes, monolithic ceramic capacitors having satisfactory relative resistivity and excellent dielectric characteristics have been desired that can be baked even in neutral or reducing atmospheres having a low oxygen partial pressure without making the ceramic materials therein into semiconductors. As materials that meet such requirements, proposed were compositions of the type of BaTiO.sub.3 --CaZrO.sub.3 --MnO--MgO, such as those disclosed in Japanese Patent Application Laid-Open No. 62-256422; compositions of the type of BaTiO.sub.3 --MnO--MgO-rare earth oxide, such as those in Japanese Patent Application Laid-Open No. 63-103861; and compositions of the type of BaTiO.sub.3 --(Mg,Zn,Sr,Ca)O--Li.sub.2 O--SiO.sub.2 --MO (MO:BaO, SrO, CaO), such as those in Japanese Patent Publication No. 61-14610.

However, in the non-reducible, dielectric ceramic compositions disclosed in Japanese Patent Application Laid-Open No. 62-256422, CaZrO.sub.3 and also CaTiO.sub.3 that may be formed during the baking step often give heterogeneous phases with Mn and others, which lowers the reliability of the capacitors at high temperatures.

In the non-reducible, dielectric ceramic compositions disclosed in Japanese Patent Application No. 63-103861, the grain sizes of the crystals of the essential component, BaTiO.sub.3 have a great influence on the insulating resistance and the temperature-dependent capacity of the capacitors. Using the compositions, therefore, it is difficult to control the grain sizes of the crystals in order to obtain capacitors with stable characteristics. Moreover, the insulating resistance in terms of its product with the capacitance (product CR) of the capacitors disclosed is from 1000 to 2000 (.OMEGA..multidot.F), which is hardly within the practicable range.

The non-reducible, dielectric ceramic compositions disclosed in Japanese Patent Publication No. 61-14610 give dielectric layers having a dielectric constant of from 2000 to 2800, which is lower than that of from 3000 to 3500 in the conventional, dielectric ceramic compositions combined with noble metals such as Pd. Therefore, if the compositions disclosed are directly substituted for the conventional materials in order to reduce the cost of the capacitors, such is disadvantageous and problematic in the viewpoint of the desired small-sized, large-capacity capacitors.

In addition, the dielectric layers to be produced from the non-reducible, dielectric ceramic compositions previously proposed tended to have noticeably lowered resistance values at high temperatures, even though having high insulating resistance at room temperature. Since this tendency was great especially in strong electric fields, it was a great bar to the production of thin dielectric layers. For these reasons, monolithic ceramic capacitors having thin dielectric layers made from such non-reducible, dielectric ceramic compositions have not heretofore been put to practical use. In addition, the non-reducible, dielectric ceramic compositions were defective in that their reliability in high-temperature and high-humidity conditions (that is, the moisture-resistant load characteristic) was lower than that of the conventional materials combined with inner electrodes of Pd or the like.

In order to solve the above-mentioned problems, we, the present inventors have already proposed novel, non-reducible, dielectric ceramic compositions such as those in Japanese Patent Application Laid-Open Nos. 05-009066 to 05-009068. In today's market, however, capacitors having much better characteristics, especially those in high-temperature and high-humidity conditions are required. Therefore, it has become necessary to propose dielectric ceramic compositions with much better characteristics.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a small-sized, large-capacity, monolithic ceramic capacitor, which can be produced even through baking in atmospheres having a low oxygen partial pressure without producing semiconductors, which has a dielectric constant of 3000 or more, which has an insulating resistance of 6000 .OMEGA..multidot.F or more in terms of its product with the capacitance (the product CR), of which the insulating resistance is lowered at most slightly even in strong electric fields, which has good reliability and has a high rated voltage even when the dielectric layers thereof are thinned, which has temperature-dependent capacitance that satisfies the B-level characteristic standard stipulated in the JIS Standard and the X7R-level characteristic standard stipulated in the EIA Standard, and which has excellent weather resistance even under a load of high temperature and/or a load of high humidity.

Specifically, the present invention provides a monolithic ceramic capacitor composed of a plurality of dielectric ceramic layers, a plurality of inner electrodes formed between the dielectric ceramic layers in such a manner that one end of each inner electrode is exposed out of a end of the dielectric ceramic layers, preferably alternating ends, and outer electrodes electrically connected with the exposed inner electrodes, which is characterized in that

the dielectric ceramic layers each are made of a material comprising barium titanate having a content of alkali metal oxide impurities of about 0.02% by weight or less, and also yttrium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, manganese oxide, cobalt oxide and nickel oxide, and containing a side component, magnesium oxide in an amount of from about 0.5 to 5.0 mols, in terms of MgO, relative to 100 mols of an essential component having the following compositional formula:

(1-.alpha.-.beta.) {BaO}.sub.m .multidot.TiO.sub.2 +.alpha.{(1-x)M.sub.2 O.sub.3 +xRe.sub.2 O.sub.3 }+.beta.(Mn.sub.1-y-z Ni.sub.y Co.sub.z)O

where

Re.sub.2 O.sub.3 is one or more selected from Gd.sub.2 O.sub.3, Tb.sub.2 O.sub.3 and Dy.sub.2 O.sub.3 ;

M.sub.2 O.sub.3 is one or more selected from Y.sub.2 O.sub.3 and Sc.sub.2 O.sub.3 ; and

.alpha., .beta., m, x, y and z are as follows:

0.0025.ltoreq..alpha..ltoreq.0.025

0.0025.ltoreq..beta..ltoreq.0.05

.beta./.alpha..ltoreq.4

0<x.ltoreq.0.50

0.ltoreq.y<1.0

0.ltoreq.z<1.0

0.ltoreq.y+z<1.0

1.000<m.ltoreq.1.035,

and further containing from 0.2 to 3.0 parts by weight, relative to 100 parts by weight of said components, of an oxide of the type of Li.sub.2 O--(Si,Ti)O.sub.2 --Al.sub.2 O.sub.3 --ZrO.sub.2 ; and

the inner electrodes are made of nickel or a nickel alloy.

Preferably, the Li.sub.2 O--(Si,Ti)O.sub.2 --Al.sub.2 O.sub.3 --ZrO.sub.2 oxide falls within a compositional range as surrounded by six lines formed by connecting six points representing the mol % of:

A (20, 80, 0)

B (10, 80, 10)

C (10, 70, 20)

D (35, 45, 20)

E (45, 45, 10)

F (45, 55, 0)

in a triangular diagram of {Li.sub.2 O, (Si.sub.w Ti.sub.1-w)O.sub.2, M}, in which 0.30.ltoreq.w.ltoreq.1.0, and M is at least one selected from Al.sub.2 O.sub.3 and ZrO.sub.2, and provided that w<1.0 for the composition on the line A-F.

Preferably, the impurities are less than about 0.015%, 0.004.ltoreq..alpha..ltoreq.0.02, 0.009.ltoreq..beta..ltoreq.0.04, .beta./.alpha..ltoreq.2, x is abut 0.1-0.4, y is about 0.1-0.8, z is about 0.3-0.8, y+z is about 0.3-0.8, m is about 1.01-1.035, 0.4.ltoreq.w.ltoreq.0.9, said Li.sub.2 O--(Si,Ti)O.sub.2 --Al.sub.2 O.sub.3 --ZrO.sub.2 oxide is about 0.8-2 parts, and the grain size is less that about 1 micron and most preferable less than about 0.075 micron.

Also preferably, the outer electrodes each are made of a sintered layer of an electroconductive metal powder or of an electroconductive metal powder with glass frit added thereto. Still preferably, the outer electrodes each are composed of a first, sintered layer of an electroconductive metal powder or of an electroconductive metal powder with glass frit added thereto, and a second, plated layer formed on the first layer.

The monolithic ceramic capacitor of the present invention is advantageous in that it can be produced even through baking in neutral or reducing atmospheres at temperatures falling between 1260.degree. C. and 1300.degree. C., and has an insulating resistance of 6000 .OMEGA..multidot.F or more in terms of its product with the capacitance (the product CR), that its insulating resistance is lowered little even in strong electric fields, that it has good reliability and has a high rated voltage even when its dielectric layers are thinned, that it has a high dielectric constant of 3000 or more, that it has temperature-dependent capacitance which satisfies the B-level characteristic standard stipulated in the JIS Standard and the X7R-level characteristic standard stipulated in the EIA Standard, and that its characteristics are not worsened even in high-temperature and high-humidity conditions

Accordingly, the monolithic ceramic capacitor of the present invention can have inner electrodes of nickel or an nickel alloy. Therefore, according to the present invention, it is possible to significantly reduce the cost of the monolithic ceramic capacitors of the invention without worsening all their characteristics including the weather resistance such as high-temperature load resistance and high-humidity load resistance, as compared with the conventional, monolithic ceramic capacitors comprising noble metals such as Pd.

The above-mentioned object and the other objects of the present invention, and also the characteristics and the advantages thereof will be clarified further more in the detailed description of the preferred modes of carrying out the invention and the examples of the invention, which is made hereinunder with reference to the drawings attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating one embodiment of the present invention.

FIG. 2 is a plan View illustrating one embodiment of the first dielectric ceramic layer to be laminated.

FIG. 3 is a perspective, exploded view illustrating the lamination of first dielectric ceramic layers and second dielectric ceramic layers to construct the monolithic dielectric ceramic body to be in the capacitor of the invention.

FIG. 4 is a three-component compositional graph for {Li.sub.2 O, (Si.sub.w Ti.sub.1-w)O.sub.2, M} where M is at least one selected from Al.sub.2 O.sub.3 and ZrO.sub.2, which indicates the compositional range of the oxide additive Li.sub.2 O--(Si,Ti)O.sub.2 --Al.sub.2 O.sub.3 --ZrO.sub.2.

PREFERRED MODES OF CARRYING OUT THE INVENTION

FIG. 1 is a cross-sectional view showing one embodiment of the present invention. The monolithic ceramic capacitor 10 illustrated comprises a monolithic dielectric ceramic body 12. The monolithic dielectric ceramic body 12 is formed by integrally laminating a plurality of first dielectric ceramic layers 14a and two second dielectric ceramic layers 14b. In the monolithic dielectric ceramic body 12, the dielectric ceramic layers 14a and 14b are integrally laminated in such a manner that the two dielectric ceramic layers 14b are disposed at the both sides, while sandwiching therebetween a plurality of the first dielectric ceramic layers 14a. These dielectric ceramic layers 14a and 14b are laminated along with inner electrodes 16 alternately embedded therein. On each of both sides of the monolithic dielectric ceramic body 12, formed are an outer electrode 18, a first plate film 20a and a second plate film 20b in that order. The first plate film 20a may be made of nickel or copper, and the second plate film 20b may be made of solder or tin. Accordingly, the monolithic ceramic capacitor 10 is shaped in the form of a rectangular parallelepiped chip.

Now, a method for producing the monolithic ceramic capacitor 10 of the invention is described below in the order of the steps constituting the method.

First, the monolithic dielectric ceramic body 12 is produced as follows. As in FIG. 2, a raw material powder comprising barium titanate, yttrium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, manganese oxide, cobalt oxide, nickel oxide, magnesium oxide, and an oxide glass consisting essentially of Li.sub.2 O--(TiO.sub.2,SiO.sub.2)--Al.sub.2 O.sub.3 is formed into a slurry, and then sheeted to prepare a first, dielectric ceramic layer 14a (green sheet). On one surface of the green sheet, formed is an internal electrode 16 of nickel or a nickel alloy. To form the internal electrode 16, employable is any method of screen printing, metal vapor deposition or plating. A predetermined number of the first dielectric ceramic layers 14a each with the inner electrode 16 formed thereon are laminated together, and then sandwiched between two dielectric ceramic layers 14b with no inner electrode 16, as in FIG. 3, and these are integrated under pressure to give a monolithic laminate. Next, the resulting laminate is baked in a reducing atmosphere at a predetermined temperature to obtain a monolithic dielectric ceramic body 12.

Next, on both sides of the monolithic dielectric ceramic body 12, formed are two outer electrodes 18 that are connected with the inner electrodes 16. The material of the outer electrodes 18 may be the same as that of the inner electrodes 16. Apart from this, silver, palladium, silver-palladium alloys and others can be used as the material of the outer electrodes 18. In consideration of the use of the monolithic ceramic capacitor 10 and the site at which the capacitor 10 is used, suitable materials are selected for the outer electrodes 18. The outer electrodes 18 can be formed by applying a paste material of metal powder onto the baked, monolithic dielectric ceramic body 12 followed by baking. Alternatively, the paste material can be applied onto the non-baked body 12, and the composite is thereafter baked. After this, the outer electrodes 18 may be plated with nickel, copper or the like to form a first plate film 20a thereon. Last, the first plate film 20a is coated with a second plate film 20b of solder, tin or the like. Thus is produced the chip-type, monolithic ceramic capacitor 10 of the invention.

EMBODIMENTS OF THE INVENTION

Example 1

First, raw materials of TiCl.sub.4 and Ba(NO.sub.3).sub.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.sub.2 O.sub.4).4H.sub.2 O). This precipitate was decomposed under heat of 1000.degree. C. or higher to obtain the four types of barium titanate (BaTiO.sub.3) shown in Table 1. On the other hand, oxides, carbonates and hydroxides of the constitutive components were weighed to give a composition of 0.25Li.sub.2 O--0.65(0.30TiO.sub.2.0.70SiO.sub.2)--0.10Al.sub.2 O.sub.3 (by mol), then mixed, ground, and vaporized to dryness to obtain a powder. This powder was melted under heat at 1300.degree. C. in an alumina crucible, and then rapidly cooled to obtain a oxide glass having a mean grain size of 1 .mu.m or less.

TABLE 1 ______________________________________ Content of Impurities (wt. %) Mean Type Alkali Grain of Metal Size BaTiO.sup.3 Oxides SrO CaO SiO.sup.2 Al.sub.2 O.sub.3 (.mu.m) ______________________________________ A 0.003 0.012 0.001 0.010 0.005 0.60 B 0.020 0.010 0.003 0.019 0.008 0.56 C 0.012 0.179 0.018 0.155 0.071 0.72 D 0.062 0.014 0.001 0.019 0.004 0.58 ______________________________________

Next, prepared were BaCO.sub.3, which is to adjust the molar ratio, m, of Ba/Ti in the barium titanate, and Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Tb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, MnCO.sub.3, NiO, Co.sub.2 O.sub.3 and MgO each having a purity of 99% or more. Powders of these raw materials were mixed with the oxide glass at various compositional ratios as shown in Table 2 to prepare various compositions. Each composition was wet-milled in a ball mill along with a polyvinyl butyral binder and an organic solvent such as ethanol to obtain a ceramic slurry. This ceramic slurry was sheeted according to the doctor blade method to obtain a rectangular, ceramic green sheet having a thickness of 14 .mu.m. Next, an electroconductive paste consisting essentially of Ni was printed on this ceramic green sheet to form an electroconductive paste layer thereon in the form of an inner electrode.

TABLE 2 __________________________________________________________________________ Amount of (1-.alpha.-.beta.) (BaO).sub.m TiO.sub.2 + .alpha. {(1 - x)M.sub.2 O.sub.3 + xRe.sub.2 O.sub.3 } + .beta.(Mn.sub.l-x-z Ni.sub.y CO.sub.2)O Oxide Glass Sample Type of M Re Added No. BaTiO.sub.2 .sigma. Sc Y 1 - x Gd Tb Dy x .beta. .beta./.alpha. y z y + z m MgO (wt. __________________________________________________________________________ pts.) *1 A 0.0000 -- -- -- -- -- -- -- 0.0200 -- 0.20 0.20 0.40 1.010 1.00 0.80 *2 A 0.0100 -- 0.60 0.60 -- 0.40 -- 0.40 0.0000 0 -- -- -- 1.010 1.00 0.50 *3 A 0.0150 -- 1.00 1.00 -- -- -- 0.00 0.0450 3 0.20 0.20 0.40 1.010 1.00 0.50 *4 A 0.0100 0.20 0.60 0.80 -- -- 0.20 0.20 0.0200 2 0.30 0.20 0.50 0.990 1.00 0.50 *5 A 0.0200 0.20 0.70 0.90 -- 0.10 -- 0.10 0.0200 1 0.30 0.30 0.60 1.000 1.50 0.80 *6 A 0.0100 -- 0.70 0.70 -- -- 0.30 0.30 0.0300 3 0.20 0.20 0.40 1.010 0.30 0.80 *7 A 0.0100 -- 0.80 0.80 -- -- 0.20 0.20 0.0100 1 0.10 0.30 0.40 1.010 1.00 0.00 8 A 0.0025 -- 0.80 0.80 -- -- 0.20 0.20 0.0025 1 0.20 0.50 0.70 1.010 1.5 0.20 9 A 0.0250 0.20 0.70 0.90 -- -- 0.10 0.10 0.0500 2 0.20 0.20 0.40 1.005 0.60 0.80 10 A 0.0060 -- 0.70 0.70 -- 0.30 -- 0.30 0.0240 4 0.30 0.20 0.50 1.015 1.50 1.00 11 A 0.0100 -- 0.50 0.50 -- 0.50 -- 0.50 0.0025 0.25 0.10 0.50 0.60 1.005 2.50 0.50 12 A 0.0060 0.60 -- 0.60 0.40 -- -- 0.40 0.0090 1.5 0.80 0.00 0.80 1.010 1.50 1.20 13 B 0.0100 -- 0.90 0.90 0.10 -- -- 0.10 0.0200 2 0.00 0.80 0.80 1.010 1.20 0.80 14 A 0.0040 -- 0.80 0.80 0.10 -- 0.10 0.20 0.0080 2 0.20 0.10 0.30 1.010 5.00 1.00 15 C 0.0150 -- 0.70 0.70 -- 0.20 0.10 0.30 0.0300 2 0.00 0.00 0.00 1.035 0.50 1.00 16 A 0.0200 -- 0.70 0.70 0.10 0.10 0.10 0.30 0.0400 2 0.30 0.00 0.30 1.035 1.50 3.00 *17 A 0.0300 -- 0.80 0.80 -- -- 0.20 0.20 0.0300 1 0.20 0.10 0.30 1.020 2.00 1.50 *18 A 0.0200 -- 0.70 0.70 -- -- 0.30 0.30 0.0700 3.5 0.10 0.20 0.30 1.010 1.00 1.00 *19 A 0.0100 0.20 0.60 0.80 -- 0.20 -- 0.20 0.0500 5 0.30 0.10 0.40 1.010 1.00 0.80 *20 A 0.0200 -- 0.30 0.30 0.70 -- -- 0.70 0.0400 2 0.20 0.10

0.30 1.010 1.00 0.80 *21 A 0.0100 -- 0.80 0.80 -- -- 0.20 0.20 0.0100 1 1.00 0.00 1.00 1.010 1.20 0.80 *22 A 0.0050 -- 0.70 0.70 -- -- 0.30 0.30 0.0100 2 0.00 1.00 1.00 1.020 1.00 0.80 *23 A 0.0050 0.20 0.60 0.80 0.20 -- -- 0.20 0.0100 2 0.50 0.50 1.00 1.030 1.00 0.80 *24 A 0.0050 -- 0.70 0.70 0.30 -- -- 0.30 0.0100 2 0.10 0.10 0.20 1.050 1.00 0.80 *25 A 0.0050 0.2 0.70 0.90 -- -- 0.10 0.10 0.0100 2 0.10 0.10 0.20 1.010 7.00 0.80 *26 A 0.0160 -- 0.70 0.70 -- -- 0.30 0.30 0.0040 0.25 0.20 0.20 0.40 0.010 1.00 5.00 *27 D 0.0100 -- 0.60 0.60 -- -- 0.40 0.40 0.0300 3 0.30 0.10 0.40 1.015 1.50 1.00 __________________________________________________________________________

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.sub.2 atmosphere at 350.degree. C. whereby the binder was burnt out, and then baked for 2 hours in a reducing atmosphere comprising gases of H.sub.2, N.sub.2 and H.sub.2 O and having an oxygen partial pressure of from 10.sup.-12 to 10.sup.-9 MPa, at various temperatures shown in Table 3, to obtain sintered ceramic bodies. A silver paste was applied onto the both sides of each sintered ceramic body, and baked in an N.sub.2 atmosphere at 600.degree. C. to thereby form outer electrodes as electrically connected with the inner electrodes.

TABLE 3 __________________________________________________________________________ Temperature- Number of Baking Dependent Variation Temperature-Dependent Product CR Product CR Mean Failures Tem- Dielectric Dielectric in Capacitance Variation in Capacitance (25.degree. C.) (125.degree. C.) Life in high- Sample perature Constant Loss .DELTA.C/C.sub.20 % .DELTA.C/C.sub.25 % (.OMEGA. .multidot. F) (.OMEGA. .multidot. Time humidity No. �.degree.C.! .epsilon. tan .delta. % -25.degree. C. 85.degree. C. -55.degree. C. 125.degree. C. max 16 V 160 V 16 V 160 V (hr.) Load __________________________________________________________________________ Test *1 1280 2830 3.1 -4.8 18.0 -10.6 13.2 25.3 6100 2030 2140 510 52 0/72 *2 1300 As Semiconductors were formed, the measurement was impossible *3 1280 3360 2.0 0.1 -7.5 -0.4 -8.9 8.9 6980 2970 2740 710 352 0/72 *4 1280 As Semiconductors were formed, the measurement was impossible *5 1280 3280 2.2 -0.1 9.5 -0.5 -10.5 10.5 4500 690 1520 350 229 0/72 *6 1280 3510 2.3 1.4 -14.6 1.1 -17.9 19.3 4100 1510 1480 390 235 0/72 *7 1350 As the sample was sintered insufficiently, the measurement was impossible 8 1280 3310 2.0 0.3 -10.0 -0.5 -11.2 12.0 7250 2900 2540 660 575 0/72 9 1280 3180 1.8 0.3 -8.3 -0.5 -9.5 9.6