EP0660964A1 - MATERIAL FOR ELECTRICAL CONTACTS BASED ON STANNIC SILVER-OXIDE OR ZINC SILVER-OXIDE AND MANUFACTURING METHOD THEREOF. - Google Patents

Abstract

A material for electric contacts based on silver-tin oxide is obtained by mixing a powder of silver or an alloy mainly containing silver with a powder consisting mainly of tin oxide and 0.01 to 10 wt. % (in relation to the quantity of tin oxide) of an additive consisting of one or more compounds containing silver, oxygen and a metal from sub-groups II to VI of the periodic system and/or antimony, bismuth, germanium, indium and gallium, compacting the mixture and sintering it. The tin oxide may be replaced by zinc oxide.

Description

 Material for electrical contacts based on silver-tin oxide or silver-zinc oxide and process for its manufacture

The invention is based on a material for electrical contacts based on silver-tin oxide with the features specified in the preamble of claim 1. Such a material is known from WO 89/09478.

Because of their better environmental compatibility and their at least partially more favorable service life, contact materials based on silver-tin oxide have begun to replace the previously preferred silver-cadmium oxide materials. Since the tin oxide tends to conduct poorly under the influence of arcs due to its higher thermal resistance. de form slag layers on the contact surface, the heating behavior under constant current is unsatisfactory for contacts made of silver-tin oxide. In order to eliminate this disadvantage, it is known to add additives in powder form to the material produced by powder metallurgy, which lead to a lower temperature at the contact point. Tungsten and molybdenum oxide and carbide have been mentioned in the patent literature as suitable additives in this sense (DE-A-29 33 338, DE-A-31 02 067, DE-A-32 32 627). Bismuth and germanium oxide have also been mentioned as additives (DE-A-31 02 067 and DE-A-32 32 627). These additives help to wet tin oxide particles, so that when the contact piece surface melts locally under the action of a switching arc, the tin oxide remains in fine suspension. In addition to this positive effect with regard to the heating behavior under continuous current, these additives also have undesirable side effects. The plastic deformability of the silver-tin oxide contact materials, which is not entirely satisfactory anyway, for the improvement of which, for example, a pretreatment of the tin oxide powder is carried out by annealing (DE-A-29 52 128) is made worse by these additives because they have an embrittling effect. This applies in particular to bismuth and molybdenum oxide. A further disadvantage, in particular of the tungsten and molybdenum compounds, is that they are particularly subject to switching operation under ACI stress (DIN 57660 part 102) favor a material transfer that leads to accelerated combustion and thus to a reduction in the service life.

According to the teaching of WO 89/09478, a contact material with a low tendency to weld and the lowest possible contact temperature under constant current should be obtained by specifically producing a structure in which areas in which little or no metal oxide is present, alternate with areas in which all or the vast majority of the metal oxide component is contained in a fine distribution. For this purpose, inter alia a composite powder is produced which contains the predominant part of the tin oxide and the further oxides and / or carbides as well as part of the silver. This composite powder is mixed with the remaining silver powder and, if necessary, with the smaller remainder of the metal oxides, compacted, sintered and shaped. In this way, a material that can be used well is obtained, but by a relatively complex process. The oxides of tungsten, molybdenum, bismuth, vanadium and copper are mentioned as metal oxides.

From the essay by Christine Bourda et al. "PROPERTIES AND EFFECTS OF DOPING AGENTS USED IN AGSN02 CONTACT

MATERIALS ", published in Proc. 16th International Conference on Electrical Contacts September 7-12, 1992 in Loughborough, it is known that some oxidic additives react with silver or tin oxide; it has been found that Temperatures which are reached under the action of a switching arc in a contact material made from silver powder, tin oxide powder and molybdenum oxide powder or antimony oxide powder made from silver and molybdenum oxide silver molybdate Ag-MoO. and from silver and antimony oxide silver- Anti onat AgSbO- can arise. Regarding these two additives, however, it is noted in the literature that, after the results of experiments in this regard, they have no influence on the wettability of the tin oxide with silver, so that it is not to be expected that they will improve the heating behavior of contacts under constant current.

In the older, but not prepublished German patent application P 42 19 333.8, a material for electrical contacts based on silver-tin oxide has already been proposed, which is obtained by mixing a powder of silver or an alloy containing mainly silver with a tin oxide powder, the powder particles of which are doped with up to 5% by weight of an oxide or carbide of molybdenum, tungsten, bismuth, antimony, germanium, vanadium, copper or indium. Compacting the mixture, sintering and forming. The doped tin oxide powder is a composite powder which can be obtained by mixing the tin oxide powder with the powdery doping substance, annealing the mixture so that the doping substance and the tin oxide powder particles diffuse, and separating the excess of the doping substance from the tin oxide powder. Another method for obtaining doped tin oxide powder is disclosed in P 42 19 333.8, a solution of a salt of tin and a salt of

Spray metal or metals, from whose oxides or carbides the dopant should consist, in a hot, oxidizing atmosphere in which the salts are decomposed so that a finely divided composite powder precipitates, the particles of which contain the tin oxide and the doping substances.

The object of the present invention is to create a material of the type mentioned at the outset which, by means of additives, exhibits a heating behavior which is just as favorable as that of the known contact materials, but is more ductile and has a longer service life in the case of an ACI switch. This object is achieved by a material with the features specified in claim 1. A particularly suitable method for producing such a material is the subject of claim 9. Advantageous developments of the invention are the subject of the dependent claims.

In the powder-metallurgical production of a contact material based on silver-tin oxide, the invention additionally uses a powder which contains one or more chemical compounds of silver, oxygen and a metal from subgroups II to VI and / or antimony, bismuth, germanium , Gallium and indium, in particular silver-tungsten-oxygen compounds, silver-molybdenum-oxygen compounds, silver-antimony-oxygen compounds, silver-bismuth-oxygen compounds and silver-germanium-oxygen compounds . Although this class of compounds also includes silver antimonate and silver molybdate, from which Christine Bourda's article (see above) is known to be in a silver-tin oxide-molybdenum oxide material or Silver-tin oxide-antimony oxide material can be formed and have no favorable influence on the wettability of the tin oxide Surprisingly, with the contact material according to the invention a significantly lower heating of the contact points under constant current than with known contacts with a comparable composition in terms of quantity. It is suspected that this is related to the fact that, unlike Christine Bourda (see above), the contact material is not produced as usual by mixing and sintering silver powder, tin oxide powder and additional metal oxide powder, but rather from the outset Powder is used, which instead of a pure metal oxide such as MoO ~ a compound of the type silver metal oxygen such as Ag _^ MoO. contains, in particular if this compound is wholly or partly connected to the tin oxide powder particles, ie a composite powder is formed, in the particles of which tin oxide and the silver-metal-oxygen compound are connected to one another; this composite powder is then mixed with silver powder and sintered into a contact material.

The fact that, according to the invention, a powder is mixed with the silver powder in the powder-metallurgical production of the contact material, said powder consisting mainly of tin oxide and one or more compounds of the silver-oxygen-metal type, has surprisingly achieved decisive advantages namely, it has been shown that the contact material according to the invention achieves a certain lowering of the contact point temperature under predetermined conditions with a substantially lower proportion of the selected additive than according to the previously known prior art. First experiences with invention Contact materials according to the invention show that, according to the invention, a certain reduction in the contact point temperature can be achieved with only 1/2 to 1/10 of the additional amount required in the prior art. That applies. also for the example of molybdenum oxide, the proportion of which can be drastically reduced if it is used as silver molybdate, especially if it is bound to tin oxide particles.

This also means that the contact material is less brittle, i.e. is more ductile. Another advantage is that, due to the lower proportion of the electrically non-conductive additive, the electrical resistance of the contact material is additionally reduced, which again makes a contribution to lowering the contact point temperature.

Another advantage of the invention is that the lower proportion of the additive selected increases the service life of contact pieces made of the material, in particular under ACI test conditions. The use of the powder according to the invention surprisingly results in less burnup than in conventional silver-tin oxide contact materials with pure metal oxide additives such as tungsten oxide, molybdenum oxide or bismuth oxide.

The tin oxide particles are preferably coated at least superficially with the silver-metal-oxygen compounds. They then particularly effectively promote the wetting of the tin oxide particles with the molten liquid phase which forms under the action of an arc. One in this Modified tin oxide powder can advantageously be obtained by mixing tin oxide powder and the powdery additive with one another and annealing them together, so that the tin oxide powder particles are wetted by the additive, part of the additive also diffusing into the surface area of the tin oxide particles and therein may form a mixed oxide.

In order to provide the necessary security against welding of the contact pieces, which is required by silver-tin oxide materials, the material expediently contains 5 to 20% by weight, preferably 8 to 14% by weight, tin oxide, and thus the tin oxide can be kept in suspension as desired by the additives in the molten phase which occurs under the action of an arc, the tin oxide powder is preferably at least 0.1% by weight of the additive, but not more than 2.5% by weight , best combined with not more than 1% by weight of the additive.

Silver molybdate is particularly preferred as an additive because of its particularly favorable effect on the heating behavior.

The mixture of tin oxide and the selected additive is expediently annealed in an oxygen-containing atmosphere, preferably in air at a temperature between 500 ° C. and 800 ° C., preferably at a temperature just above the melting point of the additive, see above that it becomes liquid and the tin oxide Particles wetted superficially. The additive is then only located where its wetting-promoting effect is desired and can therefore be used sparingly. With the small amounts in which it is used, the tin oxide particles do not stick together yet; but should this happen in individual cases, you can counter this by grinding.

The tin oxide and the additive can be combined not only by joint annealing, but also by depositing the additive on the tin oxide particles using chemical or physical deposition processes.

The teaching according to the invention can be applied to

Contact materials based on silver with zinc oxide. In such materials, no additives have been used in practice so far, but efforts have been made to reduce the contact point temperature by constructive measures in the switchgear. By using a zinc oxide powder connected with an additive according to the invention, a lowering of the contact point temperature can also be achieved with this type of material.

Examples:

1.

From 100 parts by weight of tin oxide powder with a particle size of 7 μm according to FSSS (FSSS = Fisher Sub-Sieve Sizer) and 0.5 parts by weight of disilbermonomolybdate Ag-MoO. A powder mixture is produced by dry mixing of a similar or identical particle size. This powder mixture is annealed in flat ceramic dishes under air for about 1 hour at 600 ° C. and thereby the tin oxide powder with the Ag ₂ MoO. wetted. 12 parts by weight of the annealed mixture are mixed with 88 parts by weight of silver powder of about 20 μm particle size (FSSS value). The mixture is cold isostatically pressed to a block at a pressure of 200 MPa and then sintered in air at 700 ° C. for 2 hours. The sintered block is formed into a 5 mm thick band by forward extrusion. The strip is then provided with a solderable silver backing by hot roll cladding and rolled to the desired final thickness by cold rolling. If required, contact strips can be formed from this band either by chopping, punching or separating.

Second

A powder mixture is produced by dry mixing from 100 parts by weight of tin oxide powder with a particle size of 7 μm according to FSSS and 1 part by weight of silver tetra-tungstate Ag _o ^W ₄ ^0-0 _{6 of} similar or the same particle size. This powder mixture is mikschalen in flat Kera¬ annealed in air for about 1 hour at 700 ° C and thereby the tin oxide powder with the Ag _g W ₄ 0, _g be¬ networked. 10 parts by weight of the annealed mixture are mixed with 90 parts by weight of silver powder with a particle size of approximately 20 μm (according to FSSS). The mixture is cold isostatically pressed at a pressure of 200MPa into cylindrical blocks and in air at 700 ° C Sintered for 2 hours. The sintered block is coated with silver, hot placed in a reverse extrusion press and extruded through a multiple die (DE-OS 34 26 240). As a result, flat strands are obtained which have a silver surface which is readily solderable and weldable on one side. The desired final thickness is obtained by cold rolling. Contact strips can be formed from this band as required by chopping, punching or cutting.

3rd

Example 1 is modified in such a way that a mixture is prepared from 119.5 parts by weight of a tin oxide powder with a particle size smaller than 7 μm and 0.5 part by weight Ag-MoO with an average particle size of 40 μm ° C is annealed. The Ag_MoO is distributed. on the tin oxide particles. Otherwise the procedure is as in Example 1.

With contact pieces produced in this way, durability tests are carried out in a switching device with an output of 37 kW in accordance with test category AC1. After 200,000 switching cycles, the service life test was interrupted for an examination of the heating of the contact pieces with continuous current. It was found that the heating with an average of 70-90 ° K was not higher than in the case of a conventionally produced material of the composition Ag88 / Sn0 ₂ ll, 6 / MoO ₃ 0.4 with a molybdenum oxide content that is about ten times as high.

The three examples can be modified in such a way that zinc oxide is used instead of tin oxide.

Claims

Expectations: 1. Material for electrical. Silver-tin oxide-based contacts obtained by Mixing a powder of silver or of an alloy containing mainly silver with tin oxide powder and with a powder which contains 0.01 to 10% by weight (based on the amount of tin oxide) of one or more compounds as an additive, which consist of silver, oxygen and a metal from subgroups II to VI of the periodic table and / or antimony, bismuth, germanium, indium and gallium. - compacting the mixture, and - sintering. 2. Material according to claim 1, characterized in that the compounds are connected with the tin oxide to composite particles of a composite powder. 3. Material according to claim 1 or 2, characterized gekenn¬ characterized in that it contains 5 to 20 wt .-% tin oxide. 4. Material according to claim 3, characterized in that it contains 8 to 14 wt .-% tin oxide. 5. Material according to one of the preceding claims, characterized in that the additive is present in an amount of at least 0.1% by weight. 6. Material according to one of the preceding claims, characterized in that the additive is present in an amount of not more than 2.5% by weight. 7. Material according to claim 6, characterized in that the additive is present in an amount of not more than 1% by weight. 8. Material according to one of the preceding claims, characterized in that the compounds contained in the additive contain a metal from the group consisting of tungsten, molybdenum, vanadium, antimony, bismuth and germanium, molybdenum being particularly preferred. 9. A method for producing a material for electrical contacts based on Silber-Zinn¬ oxide according to any one of the preceding claims, characterized in that the powder consisting mainly of tin oxide and the additive is obtained by - Mixing the tin oxide powder with the additive in powder form and - glow of the mixture. 10. The method according to claim 9, characterized in that the mixture is annealed in air. 11. The method according to claim 9 or 10, characterized in that the mixture is annealed between 500 ° C and 800 ° C. 12. The method according to any one of claims 9 to 11, characterized in that the mixture is annealed at a temperature above the melting point of the additive. 13. The method according to any one of claims 9 to 12, by gekennzexchnet that the material according to the Sintering is densified and / or reshaped. 14. Material according to any one of the preceding claims, characterized in that the tin oxide is replaced by zinc oxide.