Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Metal and nonmetal in final product
Patent
1995-05-18
1998-10-13
Gorgos, Kathryn L.
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Metal and nonmetal in final product
419 29, 419 30, 419 45, 419 46, 419 53, 75234, 75247, 428552, 428558, 428559, 428565, B22F 312, B22F 316, C22C 105, C22C 506
Patent
active
058226745
DESCRIPTION:
BRIEF SUMMARY
This is a national stage application of PCT/EP93/02511, filed Sep. 16, 1993.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention comprises a material for electric contacts based on silver-tin oxide
2. Description of the Prior Art
Contact materials based on silver-tin oxide have begun to replace the hitherto preferred silver-cadmium oxide materials because the former are environment friendlier and generally have a longer life span. Their thermal behavior is, however, unsatisfactory, since tin oxide, when subjected to continuous current, has a tendency to produce poorly conductive slag layers on the contact surface, influenced by electric arc. In order to overcome this disadvantage, it is known that admixtures, added in powder form to the powder metallurgically produced material, bring about a temperature reduction at the contact. Suitable admixtures in this sense have been mentioned in the patent literature, especially tungsten- and molybdenum-oxide and molybdenum carbide (DE-A-29 33 338, DE-A-31 02 067, DE-A-32 32 627). Further suitable admixtures such as bismuth- and germanium-oxides have been mentioned (DE-A-31 02 067 and DE-A-32 32 627). These admixtures help wet tin oxide particles so that when the contact area melts locally under the action of electric arc, tin oxide remains suspended in fine particles. Apart from this favorable thermal behavior under continuous current, theses admixtures have also unfavorable side effects. The already somewhat unsatisfactory plastic deformation behavior, i. e. brittleness, of silver-tin oxide contact materials, which can be improved, for example, by annealing the tin oxide powder (DE-A-29 52 128), is worsened by these admixtures, since they promote embrittlement. This is particularly true for bismuth and molybdenum oxide. A further disadvantage, especially of the tungsten and molybdenum compounds, is the fact that they tend to transfer material, especially during switching operations under AC1-loads (DIN 57660, Part 102), resulting in accelerated burning off and therefore reduced life span.
According to the teaching of WO 89/09478, a contact material with low welding tendency and minimal contact temperature under continuous current can be obtained by creating a structure containing regions with little or no metal oxides, alternating with regions containing the majority of metal oxides, finely dispersed. For this purpose a powdered compound, among other means, is commercially available, containing the predominant portion of tin oxides and the other oxides and/or carbides as well as a portion of the silver. This compound powder is combined with the remaining silver powder, and with the smaller remaining portion of metal oxides, mixed, condensed, sintered and transformed. In this manner a useful material is obtained but through a relatively costly process. Mentioned metal oxides are tungsten, molybdenum, bismuth, vanadium and copper.
From the paper by Christine Bourda et al. "Properties and effects of doping agents in AGSNO2contact materials", published in Proc. 16th Int. Conference on Electrical Contacts 7.-Sep. 12, 1992 in Loughborough, it is known that many admixtures made up of oxides react with silver or tin oxide; for example it was found that in a contact material produced from silver powder, tin oxide powder and molybdenum oxide powder or antimony oxide powder, at temperatures reached during electric arcing, silver and molybdenum oxide can combine into silver molybdate Ag.sub.2 MoO.sub.4, and silver and antimony oxide can combine into silver antimonate AgSbO.sub.3. As to these admixtures, the bibliographical reference indicates that, according to results of tests, they do not influence the wetability of tin oxide and silver, so that they are not expected to improve the temperature behavior of contacts under continuous current.
In the older but not pre-published German patent application P 42 19 333.8, a material for electric contacts had already been proposed on the basis of silver-tin oxide which is obtained through mixing of a powder of silv
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Behrens Volker
Honig Thomas
Carroll Chrisman D.
Doduco GmbH & Co. Dr. Eugen Durrwachter
Gorgos Kathryn L.
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