GB1574098A - Composite powder for use in manufacturing electrical contacts - Google Patents

Abstract

In order to produce a contact piece consisting of silver and at least two oxides of base metals for electrical contacts having a small welding force, with a small amount of arc erosion at the same time, a melt consisting of silver and the base metals is processed to form a moulding by means of pressure atomising and internal oxidation, pressing, sintering and re-pressing of the atomised powder. According to the invention, cadmium, bismuth and/or lead are used as the base metals and water is used as the propulsion agent for the pressure atomisation. The proportion of base metals in the moulding is less than 20% by volume.

Description

(54) COMPOSITE POWDER FOR USE IN MANUFACTURING ELECTRICAL CONTACTS (71) We, SIEMENS AKTIENGESELL SCHAFT, a German company, of Berlin and Munich, Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to a method of producing a composite powder for use in manufacturing an electrical contact.

Contact regions of electrical contacts are preferably manufactured as moulded parts from silver since this metal has a high electrical and thermal conductivity. When contact region chatter and arc formation occur during closure of the contacts, however, welding of the contact regions results so that the contact regions can only be separated from one another by the application of a force (so-called welding force). Also, a part of the silver vaporizes in the arc (so-called burning).

It is known that the welding tendency of contact regions is reduced when the contact regions further contain one or more oxides of a base metal. Such contacts may be manufactured by spraying a molten mass of an alloy of silver and suitable base metals through a nozzle using compressed air as a propellant. The resulting alloy powder, the base metals of which have been oxidised during the compressed air spraying, is subsequently subjected to internal oxidation. From the composite powder thus produced, the contact can be formed in powder-metallurgical manner by pressing, sintering and after-pressing.

G e r m a n Offenlegungsschrift No.

2.260,559 describes a process wherein a silver alloy is sprayed under pressure and the resultant composite powder is ground.

Subsequently, suitably sized particles are sieved off and processed by pressing, sintering and after-pressing to form a contact.

As alloy constituents for the silver alloy, there are specified a number of base metals, amongst which are cadmium and lead. It is further mentioned that the silver-cad-- mium alloys may not only be sprayed with compressed air but also with compressed air and water under pressure.

Pressure spraying of metal melts using water as a propellant is used for the preparation of alloy powders (for example iron-chromium alloy powders) which con tain an oxidisable metal component- to be protected from-oxidation. However, the installations required for pressure sprayed ing by means of water are relatively expensive and are thus generally used only for those cases in which the melt to be sprayed must be protected from the admission of air.

It is further known that in pressure spraying using compressed air, generally spherical particles are formed, whilst generally diffuse particles are obtained in the presence of alloy constituents which reduce surface tension.

It has been found that the welding force and burning of electrical contacts are so correlated that measures for improving one property generally lead to a substantial deterioration in the other property.

According to the present invention, there is provided a method of producing a compostie powder for use in manufacturing an electrical contact, which method comprises (1) pressure spraying a melt comprising (i) silver, (ii) cadmium, and (iii) bismuth and/or lead using water as the propellant, and (2) subjecting the powder thus formed to internal oxidation to produce the desired composite powder; the content of (i) cadmium and (ii) bismuth and/or lead of the melt being such that the content of oxides thereof of the composite powder is less than 20% by volume.

The composite powder produced by the method of the invention can be formed into electrical contacts by pressing, sintering and after-pressing. The contacts thus produced usually have a reduced welding force and at the same time the arc burning remains low.

In the method of the invention, cadmium or bismuth and/or lead are used as base metals. The base metal content of the melt is such that the content of base metal oxides in the contact is below 20% by volume. Further, water is used as propellant for the compression spraying of the melt.

In the composite powder, the base metals are in the form of oxides. At first sight, therefore, it seems senseless to carry out expensive compressed water spraying wherein the water acts as an inert gas atmosphere for the melt which is sprayed, and then nevertheless to subsequently subject the resultant powder to oxidation.

Surprisingly, however, we have found that electrical contacts obtained from the composite powder produced in accordance with the method of the invention usually have a substantially lower welding force than contacts of the same chemical composition but obtained from a composite powder produced by compressed air spraying of the melt.

Advantageously the cadmium content of the melt is such that the cadmium oxide content of the contact is from 5 to 18% by volume. A content in this range results in a considerable reduction of the welding force without affecting the advantages which result from the use of silver.

The content of bismuth and lead of the melt is advantageously such that together the oxides are present in the contact in a quantity of from 0-1 to 5% by volume.

Even very small quantities of these base metals considerably reduce the surface tension of the melt. The resultant composite powder thus generally has bore diffuse particles. This structure can be traced in the cross-grain in the finished contact, those produced from the composite powder produced in accordance with the invention having a more uniform structure than contacts of the same chemical composition but produced from airsprayed or air/water-sprayed internally oxidised powder.

In comparison with customary manufacturing processes, the method according to the invention has the advantage that the operation can be effected with a smaller throughput volume of propellant so that only small quantities of CdO vapour are formed. Since the spraying temperature of the melt is normally from 950 to 11000C and since CdO easily vaporises at these temperatures, the air-spraying method requires a very costly separating device in order to avoid environmental poisoning by cadmium-containing gases.

The method of the invention will now be illustrated by the following Examples.

EXAMPLE I A silver alloy containing 11% by weight of cadmium and 1% by weight of bismuth was formed by melting silver granules and rods of cadmium and bismuth. The melt, at 1100 C, was allowed to fall through a funnel having an outlet nozzle 12 mm in diameter into a pressure water ring nozzle.

The aperture of the pressure water nozzle was 1 mm and the nozzle angle was 7.5O (measured relative to the perpendicular). During the spraying the water pressure was 25 atmospheres. The powder formed by the spraying was collected in a water reservoir, dried and sieved off. Those particles with a particle size below 0-2 mm were internally oxidized for 4 hours at 600"C in air. The resultant Ag-CdO-Bi2.O3 powder had excellent flowing and pressing properties and could be efficiently processed on automatic single and multilayer presses. It was pressed at a pressure of 400 MN/m2 and sintered in air at 8500C for one hour. Subsequently, an afterpressing of the sintered bodies was effected at a pressure of 800 NM/m2. The sintering and after-pressing can be repeated again to achieve further compression.

In a test switch (described in the journal "Werkstofftechnik", Vol. 7, 1976), the contacts produced in this way using the after-pressing step once only, were tested by being switched on and off 1000 times with a starting current t of 100 A and a breaking current I of 1500 A. It is 99 9% probable that the welding force of these contacts is below 60 N. For comparison, a contact was produced from a composite material of the same composition produced in a metal mould or in sheet form and subjected to internal oxidation. The welding force in this case, 120 N, was approximately twice as high. In both cases, approximately the same burning value of 24 mm2 resulted.

The contacts in accordance with this Example are particularly suitable for installation in low-voltage switching apparatus, for example relays, since they result in an increase in the starting capacity.

EXAMPLE 2 A silver alloy containing 15% by weight of cadmium and 15% by weight of lead was formed by melting silver granules, rods of cadmium and bars of lead. The melt was sprayed at 1 1000C with water under pressure as described in Example 1.

As in Example 1, the composite powder formed was sieved off, internally oxidised, pressed, sintered, and after-pressed once, to form the finished contact.

With a 99 9% degree of probability, the welding force was less than 50 N and the burning value was on average 23 mm3.

As compared to internally oxidised contacts of the same composition, the welding force was lower by a factor of approximately 2 and the burning value was higher by approximately 10%.

EXAMPLE 3 A composite powder was prepared from a silver alloy containing 8% by weight of Cd, 2 5 % of weight of Pb and 1% by weight of Bi, as in Examples 1 and 2.

Particles with a particle size of less than 0-315 mm were sieved off and from these particles contacts were produced as in Examples 1 and 2. The welding force and arc burn of these contacts are similar to those of Example 2.

By using pure oxygen instead of air for the internal oxidation, the oxidation time was considerably reduced.

WHAT WE CLAIM IS:- 1. A method of producing a composite powder for use in manufacturing an electrical contact, which method comprises (1) pressure spraying a melt comprising (i) silver, (ii) cadmium, and (iii) bismuth and/ or lead using water as the propellant, and (2) subjecting the powder thus formed to internal oxidation to produce the desired composite powder, the content of (i) cadmium and (ii) bismuth and/or lead of the melt being such that the content of oxides thereof of the composite powder is less than 20% by volume.

2. A method according to claim 1, wherein the content of cadmium of the melt is such that the content of oxide thereof of the composite powder is from 5 to 18% by volume.

3. A method according to claim 1 or 2, wherein the content of bismuth and/or lead of the melt is such that the content of oxides thereof of the composite powder is from 0-1 to 5% by volume.

4. A method according to claim 1 of producing a composite powder for use in manufacturing an electrical contact, substantially as described in any of the foregoing Examples.

5. A composite powder for use in manufacturing an electrical contact, whenever produced by the method claimed in any of claims 1 to 4.

6. An electrical contact at least the contact region of which has been manufactured from a composite powder as claimed in claim 5.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. pressed, sintered, and after-pressed once, to form the finished contact. With a 99 9% degree of probability, the welding force was less than 50 N and the burning value was on average 23 mm3. As compared to internally oxidised contacts of the same composition, the welding force was lower by a factor of approximately 2 and the burning value was higher by approximately 10%. EXAMPLE 3 A composite powder was prepared from a silver alloy containing 8% by weight of Cd, 2 5 % of weight of Pb and 1% by weight of Bi, as in Examples 1 and 2. Particles with a particle size of less than 0-315 mm were sieved off and from these particles contacts were produced as in Examples 1 and 2. The welding force and arc burn of these contacts are similar to those of Example 2. By using pure oxygen instead of air for the internal oxidation, the oxidation time was considerably reduced. WHAT WE CLAIM IS:-

1. A method of producing a composite powder for use in manufacturing an electrical contact, which method comprises (1) pressure spraying a melt comprising (i) silver, (ii) cadmium, and (iii) bismuth and/ or lead using water as the propellant, and (2) subjecting the powder thus formed to internal oxidation to produce the desired composite powder, the content of (i) cadmium and (ii) bismuth and/or lead of the melt being such that the content of oxides thereof of the composite powder is less than 20% by volume.

2. A method according to claim 1, wherein the content of cadmium of the melt is such that the content of oxide thereof of the composite powder is from 5 to 18% by volume.

3. A method according to claim 1 or 2, wherein the content of bismuth and/or lead of the melt is such that the content of oxides thereof of the composite powder is from 0-1 to 5% by volume.

4. A method according to claim 1 of producing a composite powder for use in manufacturing an electrical contact, substantially as described in any of the foregoing Examples.

5. A composite powder for use in manufacturing an electrical contact, whenever produced by the method claimed in any of claims 1 to 4.

6. An electrical contact at least the contact region of which has been manufactured from a composite powder as claimed in claim 5.