Monolithic Capacitor With Non-Noble Metal Electrodes and Method of Making the Same

This invention is intended to reduce the cost of monolithic ceramic capacitors by eliminating the need for high temperature noble metal electrodes such as platinum, palladium and the like. In lieu of such electrodes, the electrode patterns are made with layers of materials indifferent to the ceramic which can be fired with the ceramic. During firing the ceramic and indifferent layers are consolidated into a dense ceramic monolith. The indifferent material is then changed to conductive electrodes, for example by chemical conversion of the material to a conductive state or by removing the indifferent material and substituting conducting electrode material.

In the drawing,

FIG. 1 is a plan view of one of the ceramic layers used in making the capacitor which has been coated with an electrode pattern of indifferent material,

FIG. 2 is a cross sectional view of the layers before firing,

FIG. 3 is a similar view after firing,

FIG. 4 is an enlarged section on line 4--4 of FIG. 3,

FIG. 5 is a view similar to FIG. 4 after reduction of the nickel to the metallic state,

FIG. 6 is a view of FIG. 5 after removal of the metallic nickel,

FIG. 7 is a view of FIG. 6 after coating of the surface of the voids left by dissolving the metallic nickel with other electrode material such as silver,

FIG. 8 is a view similar to FIG. 7 in which the voids are filled with conductive material such as metal,

FIG. 9 is a plan view of one of the ceramic layers which has been coated with another electrode pattern of indifferent material, and

FIG. 10 is a view like FIG. 9 with still another electrode pattern.

The manufacture of the capacitor starts with a layer, lamina, leaf, stratum or sheet 1 of green ceramic dielectric, for example a high K titanate. Such ceramics consist of mixtures of barium titanate with other oxides, titanates, zirconates, stannates, etc. or precursors thereof. The layer also contains temporary binders and other ingredients which aid in processing. These ceramics are well known to the art and many variations are described in the patent literature. The layer 1 has an electrode pattern, deposit, stratum or layer 2 which extends to one edge 3 and is margined inward from the other edges to provide an insulating border. The layers 1 are stacked one on top of the other with alternate layers turned end for end as shown in FIG. 2. The stacked layers are then pressed together and fired or sintered into a monolith, unitary body or matrix as shown in FIG. 3. The firing temperatures are high, 1000.degree.-1400.degree. C. The thickness of the layer 1 depends on the voltage rating and may be from 1 to 3 mils or more. In the prior art procedures, the electrode patterns have been formed of noble metals such as platinum, palladium, etc. which withstand the high firing temperature in oxidizing atmospheres needed to optimize the properties of titanate dielectrics. Instead of the high temperature metals, the electrode patterns 2 are of a material which remains in place and is indifferent to the ceramic at its sintering temperature and is convertible to a conductive material. For dielectrics which are sintered in air or an oxidizing atmosphere, the indifferent material may be a base metal oxide such as nickel oxide either alone or mixed with compatible metal oxides such as FeO, CoO, MnO, CrO, V.sub.2 O.sub.5, SnO.sub.2, CuO, Bi.sub.2 O.sub.3, etc. The indifferent material is applied as a paint and the vehicle in which the material is suspended is vaporized or burned during the early stages of the firing. After firing the layer of indifferent mayerial may have a thickness of 2/10 mil or less. If the firing is in an oxidizing atmosphere, the indifferent material may be wholly or partially metal since the oxidizing atmosphere converts the metal to the oxide form. In FIG. 4, which is a diagrammic section of a fired monolith showing a nickel oxide layer 2 sandwiched between two titanate ceramic layers 1, the boundaries 5, 6 between the nickel oxide and the titanate ceramic are sharp and well defined. This monolith is non porous throughout. The ceramic layers are uniformly supported by the nickel oxide layers. Porosity of the nickel oxide layer can be tolerated.

Several procedures are available for converting the monolith at the stage of FIG. 4 to a usable capacitor. FIG. 5 shows the condition of the monolith after being subjected to low temperature reduction in a hydrogen atmosphere. At the low temperature, the hydrogen reduces the nickel oxide to metallic nickel but only slightly reduces the titanate ceramic. For example, at a temperature of 280.degree. C., in 24 hours the nickel oxide is reduced to porous metallic nickel as shown at 7, which forms a good capacitor electrode. At this low temperature there is some reduction of the titanate which affects the dielectric and insulating properties.

The reduction, however, is only partial and is not sufficient to destroy the utility of the capacitor. For example, with a titanate ceramic capacitor dielectric having a normal K of 6000, the low temperature reduction may reduce the K as much as 10 or 15% and also lower the d.c. insulation resistance one order of magnitude which does not impair the use as a capacitor. The power factor also remains at an acceptable 2%. The reduction of the nickel oxide is a time-temperature reaction, the lower the temperature, the longer the time. By adding to the nickel oxide other oxides such as tin oxide in small proportions, 1% or less, the reduction of the nickel oxide to the metallic state at low temperatures can be speeded up or the reduction temperature can be lowered.

Another procedure for converting the monolith of FIGS. 3 and 4 to a usable capacitor is shown in FIGS. 5, 6 and 7. FIG. 5 shows the nickel oxide reduced to porous metallic nickel. Because of the succeeding steps illustrated in FIGS. 6 and 7, there is no need for low temperature reduction so that the reduction is carried out at high temperatures which not only cause the reduction to metallic nickel but also cause reduction of the titanate ceramic to the semiconductor state. After reaching the FIG. 5 state, the metallic nickel is removed by dissolving in a solution indifferent to the ceramic, for example in dilute sulfuric acid. The ceramic body is reoxidized by firing in an oxidizing temperature such as air, restoring the ceramic to the dielectric state having its original dielectric properties. This leaves a void or slot 8 in each location previously occupied by the nickel oxide powder. The slot 8 will ordinarily be a few tenths of a mil thick while the dielectric layers 1 will ordinarily have thicknesses of from one to three or four mils. The manufacture is complete by filling the slots or by coating the surfaces 9, 10 of the slot 8 with suitable electrode material. This, for example, could be silver paint introduced into the slots 8 by capillary action or by a combination of capillary action and pressure. Many conductive paints are known, some consisting of metal pigments which form a conductive coating and others having metal compounds which break down into metallic coating. Fusible metal such as solder may be used as shown in FIG. 8.

The indifferent material 2 is not limited to oxides or chemical compounds. When the monolith is sintered in an inert atmosphere, oxidizable materials such as carbon may be used for the layers 2. Carbon is suspended in a vehicle similar to that used for nickel oxide. After firing in an inert atmosphere to mature the ceramic, refiring in an oxidizing atmosphere will remove the carbon and supply any oxygen deficiency in the dielectric. The voids left by the removal of the carbon may be filled with conductive material as shown in FIGS. 7, 8 as described above.

FIGS. 9 and 10 show layers 1 of green ceramic with electrode patterns 2 of indifferent material which are open at both ends to facilitate filling by eliminating the need for venting air during filling or for evacuating air before filling the voids with liquis conductive material. The layers 1 of FIGS. 9 and 10 may be stacked and processed in the manner shown in FIGS. 1-8.

Although the invention has been described in connection with titanate ceramics, it is advantageous in other ceramic dielectrics, particularly those requiring high firing temperatures which can be withstood only by the high temperature metals such as platinum, palladium, etc. The materials of the electrode pattern 2 should remain in place and be indifferent to the ceramic dielectric at the firing temperature required to mature the ceramic, should not melt or sublime at the firing temperature and should be convertible to a conductive state either by chemical conversion in situ or by removal to provide voids for receiving conductive materials. The indifferent material 2 supports the green ceramic 1 during the initial firing or sintering to mature the ceramic. Thereafter the ceramic is dimensionally stable and does not require such support during the subsequent firing in reducing or oxidizing atmospheres.

By way of example, and not of limitation, carbon, nickel oxide are non-metallic materials.

"Metallic"--of the nature of metal; as a metallic substance, exhibiting the characteristic properties of a metal in the free state (that is, not combined as in oxides or salts); as metallic lead.

"Nonmetallic"--not metallic.