DE3232627A1 - Material for electrical contacts - Google Patents

Abstract

Materials for electrical contacts, in particular for use in low tension switching equipment and relays, based on Ag/SnO2 are described which additionally can also contain a relatively small proportion of high-melting metal compounds, namely vanadium oxide, molybdenum oxide or bismuth titanate and preferably also molybdenum carbide or tungsten carbide. In combination with molybdenum carbide or tungsten carbide, tungsten oxide can also be employed instead of vanadium oxide, molybdenum oxide or bismuth titanate.

Description

Description:

The invention is based on materials for electrical contacts according to Jem generic term of claim 1 or IJat; ent claim 2.

Such contact materials are mainly used for electrical contact pieces used in low voltage switchgear and relays.

Considerable efforts have been made in recent years to the most common contact material in low voltage technology, silver / cadmium oxide, with regard to its abrasion resistance in the shift light, arc to improve the Reduce the need for silver for the contact pieces. Beside one looked for reasons environmental protection the cadmium in this material through an environmentally friendly Replace metal.

For the two common manufacturing methods of Ag / CdO materials, the internal oxidation of an Ag / Cd alloy or the powder metallurgical process, a number of alternative additives have been proposed, their selective advantages with regard to low burn-off and a low tendency to weld the contact pieces However, weaknesses in other important contact properties are always bought at the price of weaknesses had to.

For example, silver alloys that can be oxidized internally were used as a metallic second component already recommended tin, indium, bismuth, lead or antimony (DE-OS 24 28 146, DE-OS 24 28 147). As an oxide, mostly in combination, they can all have increased erosion resistance effect compared to silver / cadmium oxide. The concentrations required for this and precipitation forms of the oxides as grain boundary enrichment bring in conventional motor switching devices and relays too high a contact resistance and an inadmissibly strong heating of the contact pieces with itself.

In addition, these materials are almost always so brittle that they only as a contact plate in Luftschützen, but not as a rivet or profile section can be used in relays.

Extruded silver / metal oxide materials produced by powder metallurgy are usually more ductile and the choice of non-metallic components in silver can in principle be extended to nitrides, carbides and borides (H. Schreiner, Pulvermetallurgie electrical contacts, Springer-Verlag 1964, p. 12). A range of materials on this basis also show quite satisfactory results with regard to the welding behavior Results. A service life in switching operation comparable to that of silver / cadmium oxide point to practical Experience but only materials with a few few metal oxides. This includes preferably that which has long been in the patent literature known oxide of tin-IV, Sn02, (DE-PS 807 416). The burn-off of silver / tin oxide factory stoles may be obtained by aligning fibrous extrusion or drawing Oxj] lines in the silver matrix perpendicular to the contact surface are further reduced (DE-OS 2 ') 15 392). However, this form of application requires both technically how economically an unsatisfactory high expenditure for the fastening of the difficult welding or solderable contact pieces on the contact piece carriers. In addition, is because of the stability of the tin oxide the contact point temperature in motor switchgear and in relay still too high.

According to another proposed solution 3011, another addition of 0.05 to 5 wt .-% tungsten oxide to tin oxide for both silver-tin oxide materials an improvement in the erosion resistance and a reduction in the contact piece temperature cause (DE-AS 29 33 338, of which the preamble of claim 1 and 2 is derived).

It has been observed, however, that silver / tin oxide / tungsten oxide contacts the contact piece temperature not during the entire service life of the contact pieces constant was low, but after exceeding half that or increased by three quarters of the service life and was able to reach values which are otherwise observed with silver / tin oxide contact materials without tungsten oxide. It is believed that this phenomenon is due to the contact pieces gradually forming a Reduce tungsten oxide due to a significantly lower thermal stability of tungsten oxide compared to the high thermal stability of tin oxide.

In contrast, the object of the invention is to provide a material for electrical contacts based on silver / tin oxide and at least one to create a further connection of a refractory metal with a long service life and low tendency to weld in switching mode, an improved temperature behavior the contact pieces causes.

According to claim 1, this object is achieved by a contact material on a silver / tin oxide basis, which additionally contains 0.1 to 3% by weight of a further oxidic component, consisting of vanadium oxide and / or molybdenum oxide and / or bismuth titanate and preferably also 0.014 to 3% by weight of a carbide Constituent namely tungsten carbide and / or molybdenum carbide. The proportion of tin oxide should depend on the required ductility of the contact material between 4 and 18 wt .; lie and the proportion of vanadium oxide and / or molybdenum oxide and / or bismuth titanate and / or tungsten carbide and / or molybdenum carbide taken together should not exceed 6% by weight. If the proportion of tin oxide falls below 4% by weight, then the contact pieces have an excessive tendency to weld. The proportion rises of the tin oxide to more than 18% by weight, then the contact material becomes too difficult to deform. The oxidic or carbide additives provided in addition to the tin oxide are at a proportion of less than 0.1 wt .-% are not sufficiently effective and make at a proportion of more than 6 wt .-% the contact material is too hard.

After extensive experiments, the inventors were able to demonstrate that a Addition of vanadium oxide and / or bismuth titanate and / or molybdenum oxide to the silver / tin oxide material Just as effective as tungsten oxide is the contact point temperature in the specified concentration range able to lower, but this lowered contact point temperature for a larger one Can ensure the number of operating cycles as a silver / tin oxide / tungsten oxide material.

It was also found that tungsten carbide was the sole additive to silver / tin oxide only has a slight temperature-lowering effect, but the service life of the contact pieces not only with small and medium current loads, but also can extend considerably in the heavy load area. Molybdenum carbide also has a similar effect like tungsten carbide even in small quantities. The combination of an additive made of vanadium oxide and / or molybdenum oxide and / or bismuth titanate with a carbide of tungsten or molybdenum to silver / tin oxide results in the specified mixing ratios not only technically satisfactory averages, but surprisingly even brings one over that Optimization going beyond the overall effect, both in terms of service life and also the heating behavior.

The best results were found with the mixing ratios of the carbide Component to the oxidic component (except the tin oxide) between 1: 0.5 and 0.5: 1, with a total content of these carbidic and oxidic compounds refractory metals as an additive to silver / tin oxide by 1.5% by weight.

The inventors have further found that an addition of molybdenum carbide or tungsten carbide not only in connection with silver / tin oxide contact materials with vanadium oxide, molybdenum oxide and bismuth titanate, but also in conjunction with Tungsten oxide a very favorable influence on the limitation of the contact point heating and has on the contact life (claim 2), so that also with it an essential Improvement over the previously known silver / tin oxide / tungsten oxide contact materials is achieved.

The beneficial effect of the additives used according to the invention for silver / tin oxide contact materials surprised because the thermal stability of vanadium oxide, molybdenum oxide and bismuth titanate is still considerably less than that of tungsten oxide; the favorable effect is apparently based on a special interaction (of the It is believed that they are associated with the relatively low melting temperatures of the oxidic Components) between these additives and the tin oxide. This guess is based on the observation that on the one hand these additives in connection with no tin oxide containing silver / metal oxide contact materials and on the other hand Compounds of related refractory metals such as tantalum oxide or niobium oxide in connection with silver / tin oxide contact materials no comparably cheap Cause effect. Additions of niobium oxide or tantalum oxide even increase the contact point temperature and shorten the service life.

The contact materials according to the invention have a long service life and the heating of the contact pieces that occurs during switching operation remains the entire service life of the contact pieces is limited to a low value.

Furthermore, it has been shown that the addition of small amounts of a hard material like tungsten carbide or molybdenum carbide a more even distribution during processing the tin oxide and the other oxidic constituents and the otherwise necessary Complicated process steps for homogenizing the powder mixture are unnecessary. What all of the additives mentioned have in common is that they result in a better making capacity compared to contact pieces made of silver / tin oxide - or of silver / tin oxide / tungsten oxide materials to lead.

Examples of contact materials according to the invention are given below specified.

Example 1: 88.5% by weight of commercially available silver powder (grain size C 36 µm), 10% by weight tin oxide (grain size L 5 µm), 0.7% by weight molybdenum oxide (grain size c: 7.5 µm) and 0.8% by weight tungsten carbide (grain size L 2.5um) are mixed dry as a powder for 1 hour, then sieved (mesh size L 60 zum), then isostatically pressed into blocks of approx. 50 kg, at 11000K over Sintered for 1.5 hours, then reduced in area into blocks with a Cross-section of 10 x 75 mm2 hot-extruded, hot-roll clad with fine silver sheet, Hot rolled down to the final thickness and processed into contact plates using the usual methods.

Example 2: A mixture of 88.5% by weight of silver powder with 10% by weight Tin oxide and 1.5% by weight of molybdenum oxide are produced by mixed precipitation using known methods manufactured, hydraulically pressed into a two-layer block with a fine silver back, then sintered at 11500K, then with a reduction in cross-section to form two-layer profiles With a fine silver back, hot-extruded, then rolled to final dimensions and then re-sized conventional method processed into contact plates Example 3: 91.2% by weight commercially available silver powder (grain size L 63 µm), 8% by weight of commercially available tin oxide (Grain size C 5 µm) and 0.8 wt .-% tungsten carbide (grain size L 2.5 µm) are used as Powders mixed together, then isostatically made into blocks of approx. Pressed 12 kg, then sintered at 11500K, then into wires with a diameter of 3.5 mm hot extruded, drawn to a diameter of 1.4 mm and according to conventional methods processed into solid rivets.

Example 4: 88% by weight of commercially available silver powder (grain size L 36 µm), 10.5% by weight tin oxide (grain size> 5 µm), 1% by weight tungsten oxide (grain size > 10 µm) and 0.5 wt .-% tungsten carbide (grain size 2.5 µm) are dry as a powder intimately mixed with one another, then sieved (mesh size> 60um), then Isostatically pressed into blocks, sintered at 11200K for approx. 1.5 hours, under Cross-sectional reduction hot-extruded into bars with a cross-section of 10 x 50 mm2, Hot-roll clad with a fine silver sheet, hot-rolled down to the final thickness and further processed to contact platelets using customary methods.

Example 5: 88% by weight of commercially available silver powder (grain size 2 40 µm), 11.1% by weight of tin oxide (grain size> 5 µm), 0.2% by weight of vanadium oxide (grain size L 5 .mu.m) and 9.7 wt .-% molybdenum oxide (grain size> 7.5 .mu.m) become intimate as a powder mixed together, iso- statically pressed into blocks, at 11700 K approx.

Sintered for one hour, then with a reduction in cross-section Hot-extruded to two-layer profiles with silverback, then to final dimensions rolled and processed into contact plates using conventional methods.

Example 6: 88% by weight of commercially available silver powder (grain size L 140 zum), 10.5% by weight tin oxide (grain size L 5 µm), 0.5% by weight vanadium oxide (grain size C 5 .mu.m) and 0.7 wt .-% tungsten carbide (grain size C 2.5 .mu.m) become intimate as a powder mixed together, isostatically pressed into blocks, at 11300 K for about one hour long sintered, then with a reduction in cross-section to form blocks with a cross-section of 12 x 70 mm2 hot-extruded, hot-roll clad with fine silver sheet, warm Rolled down to the final thickness and processed into contact plates using customary methods.

Example 7: 88% by weight of commercially available silver powder (grain size C 40 µm), 7% by weight tin oxide (grain size t 5 µm) and 5% by weight bismuth titanate (grain size g 30 μm) are intimately mixed with one another as a powder, isostatically pressed into blocks, sintered for approx. 1 hour at 1000 ° g, then under Reduction in cross section Hot-extruded into two-layer profiles with a fine silver back, then open Rolled to the final dimension and processed into contact plates using conventional methods.

Claims (4)

"Material for electrical contacts" claims: 1. Material for electrical contacts made of silver, tin oxide and at least one other connection a refractory metal, characterized in that it is 4 to in addition to silver 18% by weight of tin oxide and 0.1 to 3% by weight of a further oxidic component, consisting of vanadium oxide and / or molybdenum oxide and / or bismuth titanate and / or 0.04 to 3 wt .-% of a carbidic component, namely tungsten carbide and / or Contains molybdenum carbide, the proportion of this further oxidic component and this carbide component together is not more than 6% by weight. 2. Material for electrical contacts made of silver, tin oxide and tungsten oxide, characterized in that it contains 4 to 18% by weight in addition to silver Tin oxide as well as 0.1 to 3 wt.% tungsten oxide and 0.4 to 3 Cew. tungsten carbide and / or molybdenum carbide contains, the proportion of tungsten oxide and tungsten carbide and / or molybdenum carbide together not more than 6 wt. 3. Material according to claim 1 or 2, characterized in that it 6 to 12% by weight of tin oxide and 0.5 to 1.5% by weight of molybdenum oxide and / or tungsten oxide and / or vanadium oxide and / or bismuth titanate and 0.6 to 2% by weight tungsten carbide, The remainder contains silver, the proportion of molybdenum oxide and / or tungsten oxide and / or Vanadium oxide and / or bismuth titanate and tungsten carbide together no longer than 2.5 wt%. 4. Material according to one of the preceding claims, characterized in that that the proportion of oxidic and carbidic constituents together is about 12 % By weight.