Compositions: ceramic – Ceramic compositions – Titanate – zirconate – stannate – niobate – or tantalate or...
Reexamination Certificate
2002-09-13
2004-07-13
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Titanate, zirconate, stannate, niobate, or tantalate or...
C501S135000, C501S136000, C501S137000, C501S138000, C501S139000, C428S403000, C428S404000, C428S405000
Reexamination Certificate
active
06762141
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a ceramic mass having a ceramic material and at least one further ceramic material which is different than the ceramic material. In addition to the ceramic mass, the invention also describes a method for the production of the ceramic mass and a use of the ceramic mass.
DESCRIPTION OF THE RELATED ART
A ceramic mass of the said type is known, for example, from WO 97/44797. The ceramic mass is part of a multilayer capacitor of thin-film design. The multilayer capacitor comprises alternating electrode layers and ceramic layers. The ceramic layers consist of different ceramic material. The joint basis of the ceramic materials is, for example, a barium-strontium titanate system. A stoichiometric composition of the barium-strontium titanate changes from ceramic material to ceramic material. Each ceramic layer inherently has its own, relatively high temperature dependency of the relative permittivity. By contrast, the overall ceramic mass of the multilayer capacitor has a relatively low temperature dependency of the permittivity, taken as a mean of the temperature dependencies of the individual ceramic materials. In this way, by way of example, a temperature change of the ceramic mass with a X7R characteristic is produced.
To obtain this characteristic of the temperature change of the ceramic mass or the multilayer capacitor, a laminated stack comprising electrode layers and green ceramic sheets with different ceramic material is sintered. During the sintering, each green sheet becomes a ceramic layer. A contact surface between two adjacent green sheets or between two adjacent ceramic layers is relatively small compared to the volume of the green sheets or of the ceramic layers. Since interdiffusion of individual elements between the ceramic layers approximately takes place only in the contact region, there is a change in the stoichiometric composition of the ceramic materials only in the region of the contact surfaces. Only here is a solid solution formed. The stoichiometric composition of the ceramic materials of the ceramic layers is substantially retained. The characteristic of the temperature change of the ceramic mass can therefore be defined substantially by the choice of ceramic materials.
A sintering temperature used during the sintering of the laminated stack is relatively high. Therefore, to produce the multilayer capacitor for the electrode layers, it is necessary to use an electrode material with a relatively high melting point. An example of a material of this type is platinum.
A process which allows the sintering temperature to be reduced, so that even a low-melting metal with a high electrical conductivity, such as silver or copper, can be fed to a sintering process, is based on the LTCC (low temperature cofired ceramic) technology (cf. for example D. L. Wilcox et al., Proceedings 1997, ISHM, Philadelphia, pp. 17-23). In this case, the ceramic mass used is low-sintering glass ceramic. Glass ceramic consists of a ceramic material and a glass material.
SUMMARY OF THE INVENTION
It is an object of the present invention to describe a ceramic mass which has a sintering temperature which is as low as possible and a certain permittivity.
To achieve the object, the invention describes a ceramic mass having a ceramic material and at least one further ceramic material which is different than the ceramic material. The ceramic mass is characterized in that at least one glass material is arranged between the ceramic materials.
The ceramic mass may be a sintered ceramic, a green product or a loose bed of the ceramic materials comprising individual ceramic particles. The ceramic mass includes at least two different ceramic materials. In particular, more than two different ceramic materials are also conceivable. By way of example, the glass material is a borosilicate glass which is mixed with alkaline-earth metal oxides. In general, any glass material which is known from the LTCC technology is suitable. In particular, a mixture of various glass materials is also conceivable. In particular, a starting material (precursor) of a glass is also suitable as the glass material. This starting material is, for example, an alkali metal alkoxide or an alkaline-earth metal alkoxide. The glass is formed from the starting material.
The basic idea of the invention consists in reducing the size of the contact surface between the different ceramic materials with the aid of the glass material arranged between the ceramic materials. In this way, the interdiffusion of individual elements between the different ceramic materials is reduced. The ceramic mass can be subjected to a sintering process, during which the formation of a solid solution takes place to a reduced extent. The stoichiometric composition of the ceramic materials is substantially retained. In this context, it is particularly advantageous if there is scarcely any contact surface between the ceramic materials. The ceramic materials are separate from one another. The formation of a solid solution is substantially suppressed. Consequently, the permittivity of the ceramic mass is also independent of a ratio of the volume of the ceramic material to its contact surface areas with adjacent ceramic materials. The temperature change of the permittivity of the ceramic mass can be easily and reproducibly set by means of the ceramic material used.
In a particular configuration, at least one of the ceramic materials includes a ceramic powder with a powder surface at which the glass material is arranged. The ceramic powder is a form of comminution of the ceramic material. The comminution is obtained, for example, by milling using a mill. The ceramic powder comprises individual powder particles (grains) and is distinguished by a powder morphology. The powder morphology is given by the shape and size of the powder particles. The shape is, for example, round or polygonal. The size results from a diameter of a powder particle. The diameter may range from 10 nm to a few &mgr;m. The powder surface is a surface of the individual powder particles. The advantage of the ceramic powder is that a homogeneous batch of the ceramic materials can be obtained, and therefore a homogeneous temperature change of the ceramic mass can be set. The temperature dependency of the permittivity is substantially equal over the entire ceramic mass. There are no local differences in the temperature dependency. This is possible in particular if not just one ceramic material but rather all the ceramic materials used are present as ceramic powders in the ceramic mass.
To prevent powder particles comprising different ceramic materials from adjoining one another, the ceramic mass may be a more or less loose batch of powder particles and the glass material in the form of glass particles. In this case, the proportion of the glass material is so great that the likelihood of powder particles of the ceramic materials being in contact with one another is as low as possible.
In one particular configuration, the glass material is a homogeneous coating of the powder surface. The homogeneous coating allows effective separation of powder particles comprising different ceramic materials. The homogeneous coating may be a single, continuous coating. In this case, the powder surface is completely covered by the coating. It is also possible for a plurality of separate partial coatings to, be present. There may be a free powder surface which is not covered by the coating. The free powder surface is dimensioned in such a way that contact between powder particles made from different ceramic materials is substantially ruled out. Furthermore, it is conceivable for the coating to be formed by glass particles with a smaller diameter than the diameter of the powder particle which adjoin one another at the surface of a powder particle and are fixedly joined to the powder surface.
As the proportion of the glass material in the ceramic mass increases, the sintering temperature drops. However, to obtain a permittivity which is as high as possible, as may be necess
Gohike Silvia
Männer Ruth
Preu Gabriele
Wersing Wolfram
Group Karl
Siemens Aktiengesellschaft
Young & Thompson
LandOfFree
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