Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Patent
1994-07-18
1996-01-23
Walsh, Donald P.
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
75233, 75234, C22C 2912
Patent
active
054862221
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to sintered composite materials for electric contacts in Dower technology switching devices, particularly for low-voltage switching devices, having a AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO composition and, if necessary, other metal oxides. In addition, the invention also relates to a process for manufacturing such a sintered composite materials.
BACKGROUND OF THE INVENTION
Materials made of silver, stannic oxide, bismuth oxide and copper oxide have proven themselves, particularly for low-voltage switching devices. These materials fully satisfy requirements with regard to useful lifetime, on one hand, and high temperature behavior, on the other hand. The useful lifetime, in particular, is determined by contact combustion loss and by welding behavior during switching operations, while the high temperature behavior is, in particular, determined by contact resistance. For example, the materials are known from EP-A-0 164 664 and EP-A-0 170 812, with AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO compositions. These references relate specifically to such compositions for which the high temperature behavior is optimized. The known materials are normally produced by internal oxidation to internally oxidized alloy powders. In this connection, other metal oxides can also be added to the internally oxidized alloy powder.
The known materials are also supposed to satisfy short-circuit current stress requirements. Specifically, for testing under short-circuit current conditions (i.e., at currents which lie significantly above the normal current stress of the network) for which VDE and IEC standards exist for an individual case, the contacts are not allowed to weld in contactors for a specified range of pre-fuses, i.e., any welding which occurs must be easily broken T.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to find a sintered composite material with a AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO composition which is improved with regard to its short-circuit current behavior.
This task is accomplished, according to the invention, in that in a material of the type stated above, the additional metal oxide mixed in separately is stannic oxide and/or iron containing at least one element of the sixth sub-group of the periodic table. In the sintered composite material according to the invention, the additional metal oxide is, in particular, ferric tungstate. Preferably, a combination of added stannic oxide and ferric tungstate is present.
It is true that a sintered composite material which consists of silver, stannic oxide, bismuth oxide and copper oxide, which additionally contains tungsten oxide and/or molybdenum oxide as a sublimating metal oxide additive, as well as a process for producing this material are all ready known from EP-B-0 118 717. Here, the stannic oxide, the bismuth oxide and the copper oxide are precipitated as globular precipitates in silver in structural regions up to maximum 200 .mu.m diameter. Whereas the sublimating metal oxide additive of tungsten oxide and/or molybdenum oxide is distributed at the surfaces of the border areas of these silver regions. This material is produced in that an alloy powder of AgSnBiCu with a predetermined composition is internally oxidized to form an AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO composite powder, the composite powder is mixed with WO.sub.3 and/or MoO.sub.3 powder in a predetermined amount, in a stirrer mill, in the presence of acetone, and the WO.sub.3 and/or MoO.sub.3 powder particles are distributed on the surface of the composite powder particles of AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO.
However, the above known materials still do not fulfill the above shorten requirements in an adequate manner. In contrast, the short-circuit behavior of the known sintered composite AgSnO.sub.2 Bi.sub.2 O.sub.3 CuO materials is surprisingly improved by the addition of separate SnO.sub.2 and/or FeWO.sub.4, which is particularly evident in the non-welding behavior of these materials in the event of short-circuit, even when using higher fuse cur
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Greaves John N.
Siemens Aktiengesellschaft
Walsh Donald P.
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