Vacuum Switch With Contact Material Containing A Minor Percentage Of Aluminum

Abstract

A vacuum switch having a pair of contact elements which are movable together characterized by the material of the contact elements containing an alloy of a metal with a high electrical conductivity, a metal which counteracts welding, and aluminum in an amount up to 1 percent by weight. In the preferred embodiments, the contact element is preferably made from a body of porous sintered tungsten which has pores that are impregnated with the alloy which comprises a copper alloy containing 0.5 to 1 percent by weight bismuth and 0.2 to 0.4 percent by weight aluminum.

Description

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The present invention is directed to a vacuum switch with metal contact elements which are movable together with at least one of the contact elements made of a contact material containing an alloy of a metal which has a high electrical conductivity as the main component and a weld-preventing metal as a secondary component of the alloy.

2. Prior Art

The material which is used for contacts in a vacuum switch has to meet a number of requirements. The contact elements must be arc resistant in order to keep from burning during the operation of the vacuum switch. For breakdown currents of, for example 3kA at 6.6kV, the contact elements must have a high electrical conductivity and a high dielectric strength. Moreover, the welding of the contacts to one another should be avoided and the static forces required to separate the contacts after a short circuit load of up to 13kA should be less than 300N. Furthermore, the extinction current on separation of the contacts should be kept as low as possible and should not be more than a few amperes.

In order to provide a material which satisfies the above-described requirements, which are in some extent contradictory, various compositions have been proposed as the contact material; however, not all of these compositions have proven to be entirely satisfactory. The most suitable contact material has been found to be one which consists of a porous sintered metal body which is resistant to burning and which may be tungsten or molybdenum. The pores of the body are filled with a metal alloy having a metal of high electrical conductivity as its main component and includes a secondary component which acts to prevent welding or is an anti-welding additive. (For example German Pat. specification Nos. 1,081,950 and 1,960,769). In such an arrangement, the metal of the high electrical conductivity is preferably copper and the anti-weld additive is preferably bismuth.

In addition to the mechanical and electrical properties of the material for the contact elements, another requirement is that the gas content of the material for the element. This gas content is of essential significance for the operational behavior of a vacuum switch. Therefore, attempts have been made to keep the material of the contact elements free from both absorbed or adsorbed gases as well as free from surface impurities. A gas content of less than 20ppm (less than 20 parts of gas to a million parts of material) is desirable. In practice, this value can only be achieved with a great difficulty and with a great unreliability. In particular, one disturbing factor is the possibility of a residual oxygen content which is not released from the material of the contact element until the vacuum switch is in operation.

SUMMARY OF THE INVENTION

The present invention is directed to the production of a contact element of a material which fulfills all the conditions for a contact element in a vacuum switch and which material is relatively inexpensive to manufacture. To accomplish this, the material of the contact element includes an alloy of a metal of high electrical conductivity as its main component, metal which resists welding as the secondary component and aluminum in an amount up to 1 percent by weight. Preferably, the contact element is a porous metal body such as porous sintered tungsten whose pores are impregnated and filled with the alloy which has copper as the metal of high electrical conductivity, the second component which is a metal that resists welding is selected from the group consisting of bismuth or tellurium, and the aluminum in the amount of up to 1 percent by weight. To increase the hardness of the alloy, it may also contain in an amount up to 1 percent by weight. Preferably, the contact element which is attached to a movable plunger such as by welding or soldering consists of the alloy without the addition of the weld preventing material to facilitate bonding of the contact material onto the plunger of the contact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a vacuum switch according to the invention; and

FIG. 2 is a side view with parts in section of a stationary contact of a vacuum switch of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles of the present invention are particularly useful when utilized in a vacuum switch such as schematically illustrated in FIG. 1. The vacuum switch has a vacuum tight housing which is formed by a cylindrical insulating body 1 of glass or ceramic material and two metal end plates 2 and 3 which are sealingly attached thereto. A pair of contact elements 4 and 5 are arranged in the housing in a facing relationship. One of these contact elements 4 is attached to a stationary conductor 6 which extends through the end plate 2 and the other contact element 5 is secured to a movable conductor 7 which extends through an aperture in the end plate 3 and sealed by a metal bellows 8. The contact elements 4 and 5 are moved together and separated by movement of the conductor 7 and the contact element 5.

As illustrated in FIG. 2, the stationary contact element 4 which is produced in accordance with the present invention is formed as a porous sintered body such as tungsten having pore volume of approximately 50 percent. The pore volume has been impregnated in a high vacuum or a pure hydrogen gas atmosphere with a metal alloy having a first component comprising a metal which has a high electrical conductivity such as copper, a second component comprising a metal which resists welding and aluminum in an amount up to 1 percent by weight. For example a porous body is impregnated with a copper alloy having 0.3 to 0.9 percent by weight bismuth and 0.2 percent by weight aluminum.

The aluminum which is included in the metal allow used to impregnate the porous body of the contact element possesses the capabilities of safely bonding to any residual oxygen and thus reduce the free gas content within the vacuum switch below the desired level of 20ppm. Since the heat of formation and thus the internal binding energy of aluminum oxide is still very great even at high temperatures, the aluminum remains bonded to the residual oxygen even during the presence of arcing in the vacuum switch. For example, the corresponding formation enthalpy at 2,000.degree. C is still 150Kcal/Mol. and thus oxygen will remain in the bonded state to the aluminum and will not be released during any arching occurring during disconnection of the contact elements. The alloying additive of aluminum also results in a reduction in the grain size in the metal alloy which ensures a uniform distribution of the weld reducing or resisting secondary component. Moreover, aluminum also has a special advantage which is that it can be easily processed under high vacuum and in graphite crucibles.

It has been suggested to use an aluminum alloy in the active regions of the contact elements for a vacuum switch. For example German Pat. specification No. 1,236,630 suggested aluminum alloys such as aluminum and lead, aluminum and bismuth, aluminum and indium, aluminum and tin. However, in these alloys, the aluminum is the main component of the alloy. Furthermore, in these known contact materials for vacuum switches, a highly heat-resistant metal, such as tungsten or molybdenum, were expressly ruled out. In the present invention, the alloy with an aluminum content of up to 1 percent by weight is an impregnating material which is used to fill the pores of a porous body which is resistant to burning. In a further development of the invention, the contact element consists of a porous body whose pore volume is filled with the alloy containing the metal of high electrical conductivity, the weld reducing or resisting metal, and the aluminum.

Preferably, the metal of the high electrical conductivity is copper and the weld reducing material is selected from a group consisting of bismuth or tellurium, the latter being suitable in a concentration of 0.3 to 0.9 percent by weight. The bismuth component is well known for its weld reducing or resisting effect so that the contact element 4 has a little tendancy for welding to the contact element 5. At the same time, due to the presence of the bismuth component, the extinction current of the vacuum switch using such a contact material is very low. By the addition of aluminum, the residual gas content of the sintered body and impregnation alloy is chemically bonded during the impregnating process in a high vacuum. Since the formation enthalpy of the aluminum oxide (Al.sub.2 O.sub.3) is very great, the oxygen remains safely bonded even in the presence of arcs. Moreover, additional grain boundaries occur within the impregnation alloy due to the aluminum and these boundaries on one hand promote the chemical bonding of any residual oxygen and on the other hand result in a more uniform distribution of the bismuth and thus a further reduction in the tendency of welding of the contact elements together during arcing.

The movable contact 5 can basically consist of the same contact materials as the stationary contact 4. However, it is advisable to use a material for the contact element 5 which contains no bismuth. This is because the presence of bismuth in the impregnating alloy not only reduces the tendancy for the welding of the contact elements but consequently also impedes the solidability of the material of the contact elements. Since it is frequently desired to secure the movable contact element to the associated movable conductor 7 by a hard soldering process, the contact element 5 is therefore expediently produced from a sintered tungsten body containing an impregnation alloy consisting of an alloy of pure copper with an addition of approximately 0.2 percent by weight of aluminum.

In the vacuum switch of the present invention, the resistance to burning of the contacts can be further increased if the impregnating alloy used to fill the pores at the sintered tungsten body also contains tin in an amount up to 1 percent by weight. The tin does in fact produce a mixed crystal formation with the bismuth, but with an approximately equal concentration of tin and bismuth in the alloy, approximately 80 percent of the bismuth remains available for separation at the grain boundaries and is thus available to reduce the welding tendancy. Since the tin content makes the impregnation alloy harder, the impregnation alloy does not gradually burn out of the sintered tungsten body even under high loading of the vacuum switch.

The invention thus provides a material for contact elements for vacuum switches in which substantially all of the requirements made on the switch can be met. The advantageous properties of the contact made of tungsten-copper-aluminum-bismuth are based on the fact among others that all four metals do not form mixed crystals with one another and thus the effect of each comes into play.

As a result the inclusion of aluminum, variations in the gas content, which are unavoidable in mass production, can be overcome. The investigations have shown that the oxygen content of the material of the described composition always lies below 10 part of the gas per million parts of material and thus well below the allowable maximum limit of 20 parts of gas per million parts of material.

The material for the contact elements of the vacuum switch in accordance with the present invention is particularly suitable for the application is medium voltage vacuum switch tubes for example for electrical protection. The breakdown current strength in such cases is less than 5KA for which tungsten-containing contact material are preferred.

While particular examples have been given, it is not neccessary to make the two contact elements of the pair from different materials. If a sufficient resistance to burning of the contact elements can be ensured by some other manner, the contact elements may be made with the alloy without using the sintered tungsten body.

Although various minor modifications may be suggested by those versed in the art, it should be understood that we wish to employ within the scope of the patent granted hereon all such modifications that reasonably and properly come within the scope of our contribution to the art.

Claims

We claim:

1. In a vacuum switch having a pair of contact elements which are movable relative to each other, the improvement comprising at least one of the contact elements containing a metal alloy having a main component of a metal of high electrical conductivity, a second component of a metal for resisting welding of the contact elements together, and aluminum in an amount up to 1 percent by weight.

2. In a vacuum switch according to claim 1, wherein said contact elements comprises a porous body of material which is resistant to burning, and the pores are filled with said alloy.

3. In a vacuum switch according to claim 2, wherein said first component of a metal of high electrical conductivity is copper and wherein said second component of a metal for resisting welding is selected from a group consisting of bismuth and tellurium.

4. In a vacuum switch according to claim 1, wherein said one contact element comprises a porous, sintered body of tungsten having pores of a volume of approximately 50 percent, said pores being impregnated with said alloy and wherein said first component of the alloy is copper, said second component is bismuth in a range of 0.5 to 1 percent by weight, and said aluminum is in a range of 0.2 to 0.4 percent by weight.

5. In a vacuum switch according to claim 4, wherein said alloy further includes up to 1 percent by weight of tin.

6. In a vacuum switch according to claim 1, wherein only one of said pair of contact elements contains said metal alloy and the other contact element of said pair contains an alloy of a metal of high electrical conductivity as the main component and aluminum in an amount up to 1 percent by weight.

7. In a vacuum switch according to claim 6, wherein the other contact element containing the alloy of a metal of high electrical conductivity as the main component and the aluminum is a movable contact.