High-voltage switches with arc preventing or extinguishing devic – Arc preventing or extinguishing devices – Vacuum
Reexamination Certificate
2001-02-08
2003-11-18
Donovan, Lincoln (Department: 2832)
High-voltage switches with arc preventing or extinguishing devic
Arc preventing or extinguishing devices
Vacuum
C218S118000
Reexamination Certificate
active
06649856
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a glazed ceramic article, a metal and ceramic assembly having the glazed ceramic article, and a vacuum switch having the metal and ceramic assembly.
A vacuum switch is widely used for controlling supply of current for thereby controlling application of high voltage. The vacuum switch includes a pair of contacts disposed within an evacuated ceramic container in order to prevent generation of spark and short of discharge which may be accompanied by cutting off of supply of current for thereby attaining assured insulation. On the outer surface of the ceramic container of the vacuum switch is generally formed a glaze layer for making higher the insulator dielectric strength resistant to the short due to creeping discharge or the like. The glaze layer is also effective for smoothing the surface of the ceramic container for thereby preventing it from becoming dirty and making higher the chemical and mechanical strength.
The glaze layer on a ceramic main body is formed by applying a slurry of glaze on the surface of the ceramic main body and firing it (which firing is called glost firing). A ceramic material generally used for such a container whose insulation is important is alumina. A glaze of a high silicate glass content and of a low melting point is widely used since the glaze layer is formed on a sintered ceramic main body by glost firing at the temperature of 1000 to 1100° C.
SUMMARY OF THE INVENTION
In the meantime, the ceramic container of the vacuum switch has a metal-ceramic joining portion for attaching thereto an arc shield or the like which is disposed within the container for shielding the contacts. Such joining portion is generally formed by soldering. In this connection, since the soldering temperature is lower than the glost firing temperature of the glaze, glost firing is first carried out for attaching the glaze layer to the ceramic container and thereafter a metallic member is soldered to the glazed ceramic container. Further, for reason of requirement with respect to an assembled or completed condition of a glazed ceramic article when it is supplied from a ceramic maker to a switch maker, e.g., for the reason of requirement that the manufacturing steps up to glost firing be allotted to the ceramic maker and the steps of soldering and onward be allotted to a switch maker, the glost firing step needs be carried out previously to the soldering step.
However, for the reason, for example, that glaze having been heretofore used has its softening temperature which is close to a soldering temperature (e.g., 780° in case of widely used Ag—Cu solder), there is sometimes caused an appearance defect due to surface roughening which is considered to be caused during soldering. Further, it is liable to adhere to such a glaze layer whose surface is roughened in this manner a dirt or the like containing a metallic constituent as a major constituent due to contaminants of a furnace (for example, metal, metal oxide or the like under high steam pressure), thus being causative of lowering the insulating ability. It is presumed that such a phenomenon is caused for the reason that the surface section of the glaze layer is softened a little to allow bubbles contained in the glaze layer to be actualized or appear at the surface section. Such a phenomenon is particularly liable to occur when the soldering step is carried out under a high vacuum condition of 1×10
−6
torr or less.
It is accordingly an object of the present invention to provide a glazed ceramic article whose glaze layer is higher in softening temperature as compared with a conventional glaze layer and which is hard to deteriorate the insulation ability or the like due to surface roughening and adherence of dirt at the time soldering of, for example, metallic members.
It is a further object of the present invention to provide a metal and ceramic assembly having the glazed ceramic article of the foregoing character.
It is a further object of the present invention to provide an insulator for support of a transmission line which has the glazed ceramic article of the foregoing character.
It is a further object of the present invention to provide a vacuum switch having the metallic and ceramic assembly of the foregoing character.
To accomplish the above objects, there is provided according to an aspect of the present invention a glazed ceramic article comprising a main body made of ceramic, and a glaze layer formed on an outer surface of the main body, the glaze layer being made of a glaze comprising 60 to 74% by weight of Si when the weight percentage is calculated in terms of SiO
2
and 16 to 30% by weight of Al when the weight percentage is calculated in terms of Al
2
O
3
.
The above described glazed ceramic article is characterized in that the composition of the glaze layer is set so as to contain 60 to 74% by weight of a SiO
2
constituent which is a major constituent of a glassy substance and 16 to 30% by weight of, i.e., a large quantity of an Al
2
O
3
constituent (alumina constituent) which has a high melting point. As a result, the softening temperature of the glaze layer can be raised, thus making it possible to prevent deterioration of the appearance of the glaze layer due to its surface roughening at the time of soldering of a metallic member to the ceramic main body, particularly soldering in a high vacuum, for thereby preventing deterioration of the insulation ability due to adherence of dirt to the glaze layer. Further, even in case the glazed ceramic article is used without being soldered thereto a metallic member, it can effectively prevent roughening of the surface of the glaze layer when it is used in a high temperature atmosphere.
When the weight percentage content of Al when calculated in terms of or by conversion to Al
2
O
3
(hereinafter referred to as WAl2O3 (% by weight)) becomes smaller than 16% by weight, the melting point of the glaze layer is lowered, thus allowing the effect of the present invention to become insufficient. On the other hand, when WAl2O3 exceeds 30% by weight, the glost firing temperature becomes too high, thus inevitably increasing the manufacturing cost. On the other hand, when the weight percentage content of Si when calculated in terms of SiO
2
becomes smaller than 60% by weight, there may possibly occur such a case wherein the glaze layer cannot obtain a sufficiently high strength and insulation ability. Further, when WSiO2 exceeds 74% by weight, the flowability of the glaze layer becomes insufficient and it may possibly become difficult to raise the melting point of the glaze layer sufficiently. In the meantime, it is more preferable that WAl2O3 ranges from 17 to 23% by weight, and it is more preferable that WSiO2 ranges from 67 to 72% by weight.
The glaze layer of the glazed ceramic article of this invention may contain secondary constituents other than Al and Si so long as the above described effect is not deteriorated. Particularly, in order to adjust the melting point (or softening temperature) of the glaze and make higher the smoothness or flatness of the glaze layer, which is attained by applying a suitable fluidity to the glaze layer at the time of glost firing, the glaze layer may contain proper quantities of alkali metal constituents (particularly, Li, Na, K) or alkali earth metal constituents (particularly, Ca). In any event, it is desirable to adjust the composition of the glaze layer so that the melting point of the glaze is within the range from 1100 to 1400° C., whereby to effectively prevent the surface roughening of the glaze layer and adherence of dirt to same which are otherwise caused at the time of soldering and prevent an excessive rise of the glost firing temperature.
The melting point of the glaze layer is herein defined as a liquidus temperature. The liquidus temperature of the glaze layer formed on the ceramic main body is determined by a heat analysis of a specimen of the glaze layer. Namely, the specimen is prepared by separating a glaze layer from a ceramic main body and subj
Inagaki Atsushi
Makino Yusuke
Donovan Lincoln
NGK Spark Plug Co. Ltd.
Sughrue & Mion, PLLC
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