Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
2001-02-07
2003-04-01
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S446000, C428S336000, C427S489000, C427S490000
Reexamination Certificate
active
06541118
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an insulator with a molding made of ceramic and a hydrophobic coating applied to the surface of the molding.
An insulator with a molding made of ceramic is used variously in electrical insulating engineering. For example, such an insulator is used as a component in microelectronics, as an insulating housing for components in power electronics, but also as a high-voltage insulator for routing overhead power lines or for keeping them apart.
A ceramic is understood as meaning a clay ceramic, a porcelain or a steatite. The ceramic is produced from the starting materials kaolin, quartz, clay, alumina and/or feldspar by mixing the same while adding various substances in a subsequent firing or sintering operation.
The versatile use of an insulator with a molding made of ceramic in electrical insulating engineering is attributable to the specific properties of the ceramic or the ceramic material which cannot be achieved by other materials. For instance, a ceramic is distinguished by high dimensional stability, great hardness and mechanical strength, by a high electrical insulating capability, by advantageous dielectric behavior, by a great corrosion resistance as a result of high resistance to chemical influences and by a great resistance to heat and effects of the weather.
In long-term use, an insulator is subject to a greater or lesser degree of superficial soiling, depending on the location at which it is used, which can considerably impair the original insulating characteristics of the clean insulator. Such soiling is caused for example by the depositing of industrial dust or salts or the separating out of dissolved particles during the evaporation of moisture precipitated on the surface. This is referred to as surface pollution.
A fired ceramic is distinguished by relatively high surface roughness. Since a rough surface soils much more quickly than a smooth one, it is known to provide the surface of the ceramic molding of an insulator with a surface glaze in the form of a vitreous melt. It is attempted in this way to achieve a kind of self-cleaning effect, which considerably reduces the soiling tendency of the insulator. However, the production costs are increased considerably by the application of the glaze. Raw materials, pigments and the preparation and application of the glaze to the sometimes complicated geometries of the ceramic moldings represent a considerable cost factor. The application of the glaze, as an additional process step, also increases the wastage from production.
Many times, the application of a smooth glaze to the surface of the ceramic molding is not sufficient to ensure the electrical characteristics of the insulator on a long-term basis. Since even a smooth glaze cannot permanently prevent deposits, the geometry of the ceramic molding must additionally be designed in such a way that the leakage path for a possible discharge current over the surface of the molding is as long as possible. Thus, for example, a high-voltage insulator has a large number of plate-shaped ribs or shields along a cylindrical shank. Allowance is made for the different locations at which it is used by different numbers of shields, differences in shield inclination and/or differences in shield projection. This configuration has the effect that the leakage path between the two poles to be insulated is increased considerably in comparison with a purely cylindrical insulator. The shield configuration in combination with the smooth glaze allows a kind of self-cleaning effect of the surface of the molding to be achieved by the soiling being washed away by rain.
In comparison with a simple form of the insulator, however, every change in the geometry toward an increased leakage distance means extra expenditure in terms of material and production time and consequently an increase in production costs.
Furthermore, it has been found that even a great leakage path for an insulator with a ceramic molding with a glaze is not always adequate to ensure the desired electrical insulating capability over a prolonged period of time under particular operating conditions. For instance, the glazed ceramic molding of an insulator which is used in cases of high surface pollution must be manually cleared of deposits at regular time intervals in order for the functional capability not to be at risk. In addition, the known glazes, consisting of a vitreous melt, display a disadvantageous hydrophilicity of their surface. A film of water which traps the dirt particles on the surface is formed. The surface of the insulator becomes conductive. As a result, so-called discharge currents develop on the moist, soiled surface, increase until there is a flashover and in this way initiate the electrical failure of the insulator.
To solve the problem, it is known from “Elektrotechnische Zeitschrift—A”, volume 96 (1995), pages 126 to 128, to apply a coating of silicone additionally to the glaze of the ceramic molding. This takes place by applying a silicone paste or a silicone elastomer. Since silicone is hydrophobic, the surface structure of the glaze is changed in such a way that it repels water. This prolongs the operating characteristics of the soiled insulator.
However, a coating of silicone paste is disadvantageously not durable, and must be renewed from time to time, for example when the system is not in operation. In addition, both the necessary silicone paste and the silicone elastomer are expensive.
Furthermore, the publication “Insulators Glaze Modified by Plasma Processes”, Tyman, A.; Pospieszna, I.; Iuchniewicz, I.; 9
th
International Symposium of High Voltage Engineering, Graz, Aug. 28 to Sep. 1, 1995, discloses an insulator with a molding made of ceramic and a glaze applied to the ceramic, with a hydrophobic, plasma-polymer coating being additionally applied for the protection of the glaze from external influences. Disadvantageously the hydrophobicity and durability of the plasma-polymer coating described are strongly dependent on the type of glaze.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an insulator that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, with a molding made of a ceramic which has high long-term stability with regard to its electrical insulating capability, in particular when used in a damp and/or dust-containing environment.
With the foregoing and other objects in view there is provided, in accordance with the invention, an insulator having a molding made of a ceramic and a hydrophobic coating applied to the surface of the molding, a plasma polymer being applied directly to the ceramic as the hydrophobic coating.
In other words, the insulator according to the invention is distinguished by the fact that, instead of a hydrophilic glaze, a hydrophobic plasma polymer is applied directly to the ceramic of the molding. The glaze of the surface of the ceramic molding is no longer needed and is therefore omitted. With the glaze omitted, the molded ceramic of the insulator according to the invention can have a rough surface to which the hydrophobic plasma polymer is directly applied.
Accordingly, the insulator according to the invention consists essentially of a molded ceramic having applied to a surface thereof a hydrophobic coating comprising a plasma polymer.
Previous considerations concerning the improvement of the long-term stability of the electrical insulating properties of an insulator with a molding made of a ceramic were aimed at coating the already waterproof surface of the ceramic with a smooth glaze. The glaze as such was considered an indispensable part of the ceramic molding or insulating body because of the better self-cleaning effect intended by it. For further improvement, it was attempted to compensate for the hydrophilic character of the glaze by a hydrophobic coating applied to the glaze.
In a way surprising to a person skilled in the art, the invention now envisages dispensing completely with the glaze of the
Baalmann Alfred
Hennemann Otto-Diedrich
Liebermann Johannes
Vissing Klaus
Dawson Robert
Feely Michael J
Greenberg Laurence A.
Locher Ralph E.
Siemens Aktiengesellschaft
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