EP0440620B1 - Semifinished product for electrical contacts, made of a composite material based on silver and tin oxide, and powder metallurgical process for producing it - Google Patents

Abstract

A semifinished product for electrical contacts, made of a composite material based on silver and tin oxide, has a microstructure in which regions containing little or no metallic oxide alternate with regions in which all or most of the metallic oxide component is finely distributed. Also described is a powder metallurgical process for producing the product.

Description

Technical field:

The invention relates to a semi-finished product for electrical contacts made of a composite material based on silver-tin oxide and to a powder metallurgical process for its production.

State of the art:

The invention is based on a method with the features specified in the preamble of claim 1 or a semi-finished product with the features specified in the preamble of claim 14.

Contact materials based on silver-tin oxide currently have the best prospect of replacing the tried and tested contact materials based on silver-cadmium oxide, which have been discredited due to the toxicity of cadmium. The great importance that silver-cadmium oxide contact pieces have achieved in low-voltage switchgear, especially in motor contactors, is due to the fact that they optimally combine the long service life, low welding tendency, consistently low contact resistance (and thus low contact heating), good arc quenching and good workability . Today's contact pieces based on silver-tin oxide are closest to the contact pieces made of silver-cadmium oxide in the combination of their properties, but do not yet achieve such favorable properties in all of the above-mentioned points at the same time.

It is known (DE-26 59 012 B2) that the finest possible distribution of the metal oxides in the silver matrix leads to favorable contact properties. Silver-cadmium oxide materials are therefore often produced by internal oxidation of a silver-cadmium alloy. However, semi-finished products made of silver-tin oxide cannot generally be produced by internal oxidation of a corresponding workpiece made of a silver-tin alloy, since complete oxidation of the tin located inside the workpiece is hindered by the formation of passive layers, so that the oxidation is practical is limited to a surface layer. The addition of further oxidizable metals, namely indium or bismuth, can largely suppress the formation of a passivating layer (DE-A 29 08 923). Contact pieces formed from such materials can be superior to silver-cadmium oxide contact pieces under AC3 and AC4 test conditions (defined in IEC standard 158-1), but show more contact heating in the switchgear, which can affect the life of the switchgear . In addition, the internally oxidized contact pieces can no longer be deformed.

It is also known to manufacture contact materials from silver-tin oxide by powder metallurgy, namely by mixing a silver powder with a tin oxide powder, forming silver-tin oxide blanks by pressing and sintering the powder mixture, and shaping the blanks by extrusion or by extrusion and rolling. Compared to a silver-cadmium oxide contact material, such a material produced by powder metallurgy, if it also contains small amounts of tungsten oxide or molybdenum oxide, can perform approximately equally well in contact heating and better in the AC4 service life test, it performs better in the AC3 service life test however worse off. Forming the blanks by rolling or extrusion is difficult, however, because the tin oxide particles in the silver-tin oxide composite material hinder its plastic deformation extremely. To make matters worse, the more finely the tin oxide is dispersed in the material, the more difficult it is to process the silver-tin oxide, because the more effectively the tin oxide particles hinder the plastic deformation of the composite material during mechanical shaping. In order to counteract the poor processability, it is therefore proposed in DE-A 29 52 128 to anneal the tin oxide powder at 900 ° C. to 1600 ° C. before mixing with the silver powder, as a result of which the tin oxide powder particles are coarsened and the subsequent mechanical shaping of the composite material is less hinder. However, the better processability is bought with a partial deterioration in the switching properties of the contact pieces, because the tin oxide is no longer as finely distributed in the composite material as it used to be.

Semi-finished products for electrical contacts, which consist of a powder-metallurgically produced composite material based on silver-tin oxide with the addition of at least one further metal oxide (molybdenum oxide, tungsten oxide, bismuth titanate) and a carbide component (tungsten carbide and / or molybdenum carbide), are from DE- 32 32 627 C2 known.

From EP 0 170 812 A2 it is known to melt an AgSnBiCu alloy from silver, tin, bismuth and copper, to produce an alloy powder by pressure atomizing the melt, to oxidize it internally and to form contact pieces therefrom by pressing and sintering. Compared to contacts made of silver-cadmium oxide, these contacts show approximately the same heating and have a longer service life in the AC3 test, but a shorter one longer service life in the AC4 test.

From DE-29 29 630 A1 it is known to produce a silver-tin oxide composite powder by a pyrolytic process and to form contact pieces from this composite powder by pressing and sintering. Compared to contact pieces made of silver-cadmium oxide contact pieces, these contact pieces have a longer service life, but they have more contact heating and are more difficult to process. The same applies to the contact piece known from DE 32 12 005 A1, which is produced by powder metallurgy from a silver tin oxide composite powder and has an oxide-free back made of copper or of a copper alloy. From the same DE 29 29 630 A1 it is also known to additionally store tungsten oxide or molybdenum oxide in the composite powder. This can reduce contact heating, but at the same time the service life in the AC3 test is reduced.

From DE-B-26 59 012 a powder metallurgical method for producing a contact material made of silver with two different metal oxides embedded is known, in which two silver-metal oxide composite powders are mixed, pressed and sintered, of which one composite powder only one metal oxide and the other composite powder contains only the other metal oxide.

Presentation of the invention:

The present invention has for its object to provide a semifinished product for electrical contacts based on silver-tin oxide, which can be processed well by extrusion and rolling despite a content of very small tin oxide particles and at the same time equally good in terms of life, tendency to weld and contact heating or better than semi-finished products based on silver-cadmium oxide.

This object is achieved by a method with the features specified in claim 1 and by a semi-finished product with the features specified in claim 14. Advantageous developments of the invention are the subject of the dependent claims.

The semi-finished product produced according to the invention consists of a composite material which is characterized by a special rough structure in combination with a special fine structure. The rough structure is given by the fact that in the composite material oxide-rich areas, in which all metal oxide or the vast majority of the metal oxide component is concentrated, alternate with low-oxide areas that contain only a small proportion of the metal oxide component or are even oxide-free. The low-oxide regions contain at most a small proportion of metal oxide, finely distributed in a matrix formed from the material of the first component. The oxide-rich regions contain the lion's share of the metal oxide component (in a concentration that is far higher than the usual average metal oxide concentration in a silver-tin oxide-based contact material) and the rest of the material of the first component in the manner of a penetration or intercalation composite material finely divided into one another. These areas are the result of low-oxide and high-oxide powders that have been mixed, pressed and, if necessary, sintered. The size of the low-oxide and high-oxide areas that determine the coarse structure of the composite material therefore depends on the size of the powder particles. The fine structure of the composite material is given by a finely dispersed oxide distribution in the oxide-rich areas of the composite material forming the coarse structure, possibly also in the low-oxide areas, provided that metal oxides are present in them. The entire metal oxide component is best concentrated in the composite powder used, so that the other powder, which contains the major part of the silver or the alloy mainly containing silver (first component), is completely oxide-free. In this case, areas in the composite material in which the metal oxide component is concentrated alternate with areas that are completely free of the metal oxide component. This has the advantage that the areas which contain the metal oxide component, in particular the tin oxide, are largely separated from one another by an oxide-free matrix (they "float" in an oxide-free matrix, so to speak), so that they prevent the plastic deformation during rolling or extrusion Semi-finished products hinder much less than in the case of more or less evenly distributed metal oxides over the entire material. In contrast, the semi-finished product according to the invention is characterized by improved deformability. However, this is not bought through an increased tendency to weld or through a reduced service life or through an increased electrical contact resistance.

This surprisingly favorable behavior of the contact material produced according to the invention is probably due to the fact that the contact material of known contact materials based on silver-tin oxide is not characterized by a changed total oxide content, but rather by the fact that this total oxide content has been distributed in a new way in the material, namely so that areas with a high metal oxide concentration in the material of the first component alternate with areas of low or vanishing metal oxide concentration in the material of the first component nente, the size of these areas depending on the size of the powder particles from which the composite material is made because of the powder metallurgical production. In the areas of the composite material in which the metal oxide component is present, it should be present in a very fine distribution according to the invention. The total content of the metal oxide component in the semi-finished product can and should be between 5 and 25% by weight in the usual range.

Although it is preferred to concentrate the entire metal oxide component in the one composite powder, with the result that there are areas in the semifinished product that are completely free of metal oxides and therefore the deformability of the semifinished product is particularly good, it is nevertheless possible to use a small proportion of metal oxide in to accommodate the second powder, which contains most of the silver or silver alloy; in this second powder, which can be a composite powder or a powder mixture, the content of the tin oxide and any further oxides which may be provided should not exceed 3% by weight (based on the weight of this second powder). This proportion could be added individually or as a composite powder.

Surprisingly, it has been found that contact pieces produced from the semifinished product according to the invention have a lower electrical contact transition resistance than contact pieces of the same composition which are produced in a conventional manner and thus show less contact heating, which is a further essential advantage of the invention. Presumably this is due to the fact that the tin oxide accumulates less strongly on the contacting surface in the case of contact pieces according to the invention, the tin oxide content, which is only finely dispersed in some areas, being favorable for the switching behavior, e.g. has a low tendency to weld.

In addition, it has been shown that contacts produced from the semifinished product according to the invention suffer less erosion than contact pieces of the same composition produced in a conventional manner. The service life based on the AC3 and AC4 tests is longer than with comparable AgCdO contacts.

This is also an advantage of the invention.

In order to arrive at the working material structure according to the invention with the low-oxide and high-oxide areas, the major part of the metal oxide component must be concentrated and incorporated in the composite powder. Only the relatively small proportion of metal oxide, which may still be present in the low-oxide areas of the composite, can e.g. are mixed in the form of a pure oxide powder with the powder from the first component of the material. It is preferred that the same oxides are present in the low-oxide regions as in the oxide-rich regions. The metal carbides (especially tungsten carbide and / or molybdenum carbide) that may still be present in the second component and the metals that are not dissolved in the first component (especially tungsten and / or molybdenum) can be added to the powder mixture in the form of separate powders; they are suitable in switching operation to promote the wetting of the tin oxide with silver and thereby to lower the contact transition resistance.

The composite powder can be produced by atomizing the melt of an alloy which contains metals of the first component, tin and optionally further oxidizable or non-oxidizable metals of the second component, and then oxidizing the oxidizable metals by the internal oxidation method. It is particularly advantageous to produce the composite powder by spraying an aqueous solution of salts of the metals of the first component and of tin in a hot, oxidizing atmosphere and thus pyrolytically decomposing the salts. The process, also known as spray pyrolysis, is e.g. in US-A 3 510 291, in EP-0 012 202 A1 and in DE-29 29 630C2. Metals intended for the composite powder are dissolved in a liquid and the solution is atomized in a hot reactor or into a flame, so that the solvent evaporates suddenly. The resulting solid particles react with the oxygen in the oxidizing atmosphere in the flame or in the reactor at a temperature below the melting temperature of the dissolved metals, producing powder particles in which the metals of the first component, i.e. the silver or the silver alloy, and the Metal oxide component, that is essentially tin oxide, is bound to one another in a very fine distribution. In the composite powder produced by spray pyrolysis, the metal oxide particles are mostly in sizes between 0.1 μm and 1 μm (diameter), which is advantageous for the process according to the invention. The presence of such fine metal oxide particles favors the formation of the desired properties of the contact pieces (low erosion, low tendency to weld, consistently low contact resistance), in particular if this oxide component is present in combination with an electrically highly conductive material (first component), which is the case according to the invention.

The use of composite powders produced by spray pyrolysis is also advantageous because, in particular, powder powders are obtained by spray pyrolysis Chen arise that have a spherical or potato-shaped shape, which favors the formation of a deformable semi-finished product, because the spherical or potato-shaped particles resist plastic deformation of the contact material less than irregularly jagged powder particles.

The oxidic and carbidic constituents which are optionally provided in addition to the tin oxide bring about a reduction in the contact point temperature in switching operation and an extension in the service life of the contact pieces not only in the case of small and medium current loads, but also in the heavy load range. Molybdenum carbide and tungsten carbide work in small quantities. The additional carbides and oxides should not exceed 6% by weight of the contact material so that it does not become too hard.

It may also be advantageous to add nickel to the composite material, which is insoluble in silver and is either mixed as a very fine powder with the powder formed from silver or a silver alloy, or else is also introduced as spray-pyrolytically produced silver-nickel powder.

WAYS OF CARRYING OUT THE INVENTION: 1st example

In order to produce a composite powder made of silver with 10% by weight of tin oxide and 0.3% by weight of bismuth oxide, the melt is sprayed with a corresponding silver-tin-bismuth alloy. The silver-tin-bismuth alloy powder with a particle size of less than 100 μm is internally oxidized for 6 hours at a temperature of 700 ° C. in an oxidizing atmosphere. Then 75 parts by weight of a commercially available silver powder with a particle size smaller than 40 .mu.m and 25 parts by weight of the silver-tin oxide-bismuth oxide composite powder are dry mixed for 1 hour, then isostatically pressed into blocks of approx. 50 kg weight and then sintered at a temperature of 830 _° C for 1.5 hours. The block formed in this way is placed in the recipient of an extrusion press and, with a reduction in cross section, extruded into a strand with a cross section of 10 × 75 mm ² hot, at a temperature of approx. 850 C, then hot-rolled plated with a 1.5 mm thick fine silver sheet, hot rolled down to its final thickness of 2mm and processed into contact wafers according to the usual methods.

2nd example

A silver-tin oxide composite powder with 32% by weight of tin oxide is produced by spraying an aqueous solution of silver nitrate and tin-II-chloride in a reactor heated to approx. 950 _° C. with an oxygen-containing atmosphere, whereby a silver-tin oxide composite powder fails , in the powder particles of which the tin oxide is present in a very fine distribution. Subsequently, 75 parts by weight of a silver powder with a particle size smaller than 40 .mu.m are dry mixed with 25 parts by weight of the silver-tin oxide composite powder for one hour and processed into contact wafers as in the first example. The silver-tin oxide composite in the contact wafers has a tin oxide content of 8% by weight.

3rd example

The second example is modified such that 0.5% by weight of tungsten oxide (particle size less than 10 µm) and 0.3% by weight of tungsten carbide (particle size less than 2.5 µm) are added to the powder mixture. Otherwise, the procedure is the same as in the second example. The addition of the tungsten oxide and tungsten carbide leads to a lowering of the contact point temperature and to an extended service life of electrical contact pieces produced from the semi-finished product.

4th example

A silver-tin oxide-tungsten oxide composite powder with 20% by weight of tin oxide and 0.5% by weight of tungsten oxide is produced by spraying an aqueous solution of silver nitrate, tin-II-chloride and tungsten-II-chloride in one to approx. 950 ° C heated reactor with an oxygen-containing atmosphere, whereby a silver-tin oxide-tungsten oxide composite powder fails, in the powder particles of which the tin oxide and the tungsten oxide are present in a very fine distribution. Subsequently, 50 parts by weight of a silver powder with a particle size smaller than 40 .mu.m are dry-mixed with 50 parts by weight of the silver-tin oxide-tungsten oxide composite powder for one hour and, as in the first example, processed into contact wafers.

5th example

A silver-tin oxide composite powder with 30% by weight of tin oxide is produced as in the second example. A silver-nickel composite powder with 2% by weight of nickel is produced by spraying an aqueous solution of silver nitrate and nickel-II-chloride in a heated to approx. 950 _° C. Actuator with a protective gas atmosphere (eg argon), whereby a silver-nickel composite powder fails, in the powder particles of which the nickel is present in a very fine distribution.

Subsequently, 50 parts by weight of the silver-tin oxide composite powder and 50 parts by weight of the silver-nickel composite powder are dry mixed for one hour and further processed into contact wafers as in the first example.

6th example

The fifth example can be modified in that a silver powder and a carbonyl-nickel powder are mixed with the silver-tin oxide composite powder instead of a silver-nickel composite powder. Otherwise, the procedure is as in the fifth example.

Description of the drawing:

• The attached figure shows schematically the structure of a composite material produced in accordance with the second example, in which silver-tin oxide regions 1, which are usually smaller than 50 μm, lie in a silver matrix which has arisen from the oxide-free silver powder particles.

• Erfindungsgemäß hergestellte Halbzeuge eignen sich besonders für Kontaktstücke in Niederspannungsschaltgeräten, z.B. in Motorschützen.

Industrial applicability:

• Semi-finished products manufactured according to the invention are particularly suitable for contact pieces in low-voltage switching devices, for example in motor contactors.

Claims (30)

1. A powder-metallurgical process of producing a semi-finished product made of silver and tin oxide for use in electric contacts, consisting of a composite material which consists of 60 to 95 % by weight of a first component having a high electric conductivity, namely, silver or an alloy mainly consisting of silver,

whereas the remainder consists of a second component, which is insoluble in the first component and decreases the tendency to exhibit contact welding and the contact burn-off and consists based on the weight of the composite material of 3 to 25 % by weight tin oxide, 0 to 10 % by weight of one or more further metal oxides which together with the tin oxide will be described hereinafter as the metal oxide component, 0 to 10 % by weight of one or more metal carbides and 0 to 10 % by weight of one or more further metals, which are insoluble in the first component,

wherein the tin oxide predominates in the second component and the average content of the metal oxide component is not in excess of 25 % by weight of the composite material, wherein

a composite powder which contains less than one-half of the first component and 60 to 100 % based on the metal oxide component of the metal oxide component

is mixed with one or more powders which contain the remainder of the first component and of the second component and

the powder mixture is compacted to form shaped pieces consisting of the composite material.

2. A process according to claim 1, characterized in that the shaped bodies are subsequently sintered.

3. A process according to claim 1 or 2, characterized in that the shaped bodies are subsequently deformed by coining, extruding or by extruding followed by rolling.

4. A process according to claim 1, characterized in that the entire metal oxide component is incorporated in the composite powder.

5. A process according to claim 4, characterized in that the entire second component is incorporated in the composite powder.

6. A process according to claim 1 or 4, characterized in that the further metal oxides in pulverulent form are mixed with the powder of the first component and with the composite powder of the second component.

7. A process according to claim 1 or 4, characterized in that the metal carbides in pulverulent form are mixed with the powder of the first component and with the composite powder of the second component.

8. A process according to claim 1 or 4, characterized in that the further metals of the second component in pulverulent form are mixed with the powder of the first component and with the composite powder of the second component.

9. A process according to any of the preceding claims, characterized in that the composite powder is produced in that a molten material is sprayed which has the intended content of the first component, tin and optionally further oxidizable and non-oxidizable metals of the second component, and the oxidizable metals in the alloyed or composite powder obtained by the spraying are subsequently oxidized by a process of internal oxidation.

10. A process according to any of the preceding claims, characterized in that the composite powder is produced in that a solution of salts of metals of the first component and of a salt of tin is sprayed into a hot oxidizing atmosphere, in which the salts are pyrolytically decomposed.

11. A process according to claim 10, characterized in that the solution contains also salts of the further oxidizable metals.

12. A process according to claim 11, characterized in that the solution contains salts of all oxidizable metals which are intended for the second component.

13. A process according to any of the preceding claims, characterized in that the composite powder is not in excess of 45 % by volume of the powder mixture.

14. A semi-finished product which is made of silver and tin oxide and intended for use in the manufacture of electric contacts, consisting of a composite material that consists of 60 to 95 % by weight of a first component having a high electrical conductivity, namely of silver or an alloy which mainly contains silver, and 40 to 5 % by weight of a second component, which is distributed but insoluble in the first component and reduces the tendency to exhibit contact welding and burn-off and which based on the weight of the composite material contains 3 to 25 % by weight of tin oxide, 0 to 10 % by weight of one or more further metal oxides which together with the tin oxide will be described hereinafter as the metal oxide component , 0 to 10 by weight of one or more metal carbides and 0 to 10 % by weight of one or more further metals, which are insoluble in the first component,

wherein the tin oxide predominates in the second component and the average content of the metal oxide component is not in excess of 25 % by weight of the composite material,

characterized in that the structure of the composite material comprises low-oxide regions, in which the content of the metal oxide component is 0 to 20 % of its average content and is present in a fine distribution in a matrix consisting of the material of the first component, in alternation with high-oxide regions comprising the metal oxide component in an amount of 1.5 to 6 times its average content as averaged over the semi-finished product and the remainder of the first component finely distributed one into another,

wherein the low-oxide regions and the high-oxide regions are present in the composite material in a statistically uniform distribution and a substantial part of the high-oxide regions is surrounded by the low-oxide regions.

15. A semi-finished product according to claim 14, characterized in that the low-oxide regions occupy at least 40 % by volume of the composite material and the high-oxide regions occupy the remainder of the volume of the composite material.

16. A semi-finished product according to claim 15, characterized in that the low-oxide regions occupy at least 55 % by volume of the composite material.

17. A semi-finished product according to any of claims 14 to 16, characterized in that the metal oxide component has the same composition in the low-oxide regions and in the high-oxide regions.

18. A semi-finished product according to any of claims 14 to 16, characterized in that the entire metal oxide component is concentrated in the high-oxide regions.

19. A semi-finished product according to claim 18, characterized in that the entire second component is concentrated in the high-oxide regions.

20. A semi-finished product according to any of claims 14 to 19, characterized in that the high-oxide regions are smaller than 500 x 10-⁶ m_m3.

21. A semi-finished product according to claim 20, characterized in that the high-oxide regions are smaller than 35 x 10-⁶ mm³.

22. A semi-finished product according to any of claims 14 to 21, characterized in that the first component consists of fine silver

23. A semi-finished product according to any of claims 14 to 21, characterized in that the first component is an alloy of silver and 0.1 to 10 % by weight of copper.

24. A semi-finished product according to any of claims 14 to 21, characterized in that the first component is an alloy of silver and 0.1 to 10 % by weight of palladium.

25. A semi-finished product according to any of claims 14 to 24, charcterized in that the second component contains a refractory metal in an amount of 0.1 to 10 % by weight of the entire composite material.

26. A semi-finished product according to claim 25, characterized in that the refractory metal is tungsten or molybdenum.

27. A semi-finished product according to any of claims 14 to 26, characterized in that the further metal oxides contained in the second component are selected from the group consisting of tungsten oxide, molybdenum oxide, vanadium oxide, bismuth oxide, bismuth titanate, and copper oxide.

28. A semi-finished product according to any of claims 14 to 27, characterized in that the metal carbide contained in the second component is selected from the group comprising tungsten carbide and molybdenum carbide.

29. A semi-finished product according to any of claims 14 to 28, characterized in that the composite material contains up to 10 % by weight of nickel.

30. A semi-finished product according to claim 29, characterized in that the composite material contains less than 1 % by weight of nickel.

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