U.S. patent number 3,819,897 [Application Number 05/325,613] was granted by the patent office on 1974-06-25 for vacuum switch with contact material containing a minor percentage of aluminum.
This patent grant is currently assigned to Siemens Aktiengesellschaft. Invention is credited to Hans Bettge, Gerhard Peche.
United States Patent |
3,819,897 |
Peche , et al. |
June 25, 1974 |
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.
Inventors: |
Peche; Gerhard (Berlin,
DT), Bettge; Hans (Berlin, DT) |
Assignee: |
Siemens Aktiengesellschaft
(Berlin, Munich, DT)
|
Family
ID: |
5833683 |
Appl.
No.: |
05/325,613 |
Filed: |
January 22, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Jan 21, 1972 [DT] |
|
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2202924 |
|
Current U.S.
Class: |
200/263; 200/266;
218/132; 200/264; 200/270 |
Current CPC
Class: |
H01H
1/0203 (20130101) |
Current International
Class: |
H01H
1/02 (20060101); H01h 001/02 () |
Field of
Search: |
;200/166C,144B
;75/162 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Scott; James R.
Assistant Examiner: Smith; William J.
Attorney, Agent or Firm: Hill, Gross, Simpson, Van Santen,
Steadman, Chiara & Simpson
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.
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.
* * * * *