EP0171339B1 - Method of making electrical contacts - Google Patents

Abstract

1. A process for the production of an electrical contact formed by a substrate of carbon fibres of small diameter which are aligned in a preferential direction and impregnated with a metal or metal alloy which is a good conductor of electric current, characterized in that the carbon fibres forming the substrate are between 2 and 50 mu m in diameter and that after impregnation under pressure with a metal or alloy which does not react with the carbon, the material produced in that way is subjected to a hot extrusion operation.

Description

The present invention relates to a method for manufacturing electrical contacts made of metal or metal alloy and carbon fibers.

It is known, from French patent FR-A 2115865, to make electrical contacts based on metal and carbon by dipping carbon wires under a protective gas or under a reducing atmosphere or under a high vacuum of less than 5.10- ³ Pa (5.10- ⁵ torrs), in a good conductive liquid metal, in particular copper and / or silver, to which is added 0.5 to 5% of a metal capable of forming a carbide in order to make carbon wettable by metal.

This process has the major drawback of being difficult to implement. In particular, the duration of the soaking must be well defined so as not to transform all the carbon into carbide, and one cannot obtain contacts consisting solely of the chosen metal and of carbon wires.

Furthermore, it is often desired that the carbon wires are aligned in a preferential direction, that is to say in general perpendicular to the contact surface, so as to reduce the wear by the arc which forms during switching operations.

This alignment, within the framework of the cited patent, can only be obtained by producing an adequate carbon substrate before soaking.

It is also known from published Japanese patent application 5754236 to make an electrical contact by mixing metal powder with carbon fibers previously coated with a metal or a magnetic alloy, then by compressing this mixture in the presence of a magnetic field.

One of the drawbacks of this process is the obligation to coat the carbon fibers. This operation is expensive and delicate, and introduces into contact magnetic substances which are not always desired depending on its use.

Another drawback is that it requires an expensive and bulky device for compression.

One of the aims of the invention is to overcome the drawbacks which the methods described present.

• - Les fibres de carbone soient très courtes (de l'ordre de 15 um à 150 µm), très bien orientées et très bien dispersées,
• - la densité de carbone puisse être facilement ajustée.

Another object of the invention is to be able to make electrical contacts made of metal or of metal alloy and carbon fibers, of small diameter, of the order of 2 to 50 _I Lm, contacts, in which.

• - The carbon fibers are very short (around 15 µm to 150 µm), very well oriented and very well dispersed,
• - the carbon density can be easily adjusted.

These objects are achieved according to the invention by the process which is the subject of claim 1.

• a) l'imprégnation d'un substrat de fibres de carbone de faible diamètre avec le métal ou l'alliage fondu, sous pression,
• b) le filage à chaud du matériau ainsi obtenu.

A method of manufacturing an electrical contact consisting of metal or metal alloy and carbon fibers of small diameter of the order of 2 to 50 Rm and aligned in a preferred direction, is characterized in that it combines the two operations following:

• a) impregnation of a small diameter carbon fiber substrate with the molten metal or alloy, under pressure,
• b) hot spinning of the material thus obtained.

The two operations of this process are known in themselves, the first by French patent FR-A 1 368129, the second being commonly used in metallurgy for example.

• - ne contenant que les éléments constitutifs de départ puisque le mode d'imprégnation choisie ne nécessite aucune adjonction et que l'orientation des fibres est obtenue par le filage,
• - dont les fibres sont alignées quasiment selon une seule direction, et très bien dispersées.

Their combination for the intended application, not yet known to date, allows contacts to be obtained in a simple and economical manner:

• - containing only the initial constituent elements since the method of impregnation chosen does not require any addition and the orientation of the fibers is obtained by spinning,
• - whose fibers are aligned almost in one direction, and very well dispersed.

This process also makes it possible to use carbon fibers of relatively large length at the start of the order of 30 to 50 mm.

During spinning, they are sectioned into fragments of length on the order of 15 to 500 ILm.

The metal or metal alloy is chosen from those which are good conductors of electricity and which do not react with carbon at the melting temperature of the metal.

By way of indication, mention may be made of copper and its alloys, silver and its alloys, cadmium, etc.

The choice of metal or alloy can be dictated by reasons of economy and / or abundance of the metal. Thus, copper and its alloys can be used which, for good contact performance, have the advantage of being more economical and more abundant than silver.

The carbon fiber substrate can consist of loose fibers or preferably a felt.

Of course, carbon fibers are also understood to mean graphite fibers.

The percentage of carbon in the final product is obtained by varying the density of the fiber substrate. This density can vary from 0.05 to 1, the rate of metal or alloy therefore varies from 97.5% to 45% by volume.

The following examples, given by way of non-limiting illustration, illustrate the invention.

Example 1

80 g of carbonized viscose fibers with a diameter of 10 μm and a length of 50 mm are compacted in the form of a felt with a diameter of 80 mm and a thickness of 80 mm.

This felt is impregnated with molten copper, at 1100 ° C. under a pressure of (5 bars) 5.10 ⁺ 5 Pa.

The billet obtained contains 87% by volume of copper and its examination shows that the carbon fibers have retained their length (approximately 50 mm) and their random orientation after impregnation.

This billet is then hot-spun at 850 ° C through a circular section of 12 mm diameter.

• - les fibres de carbone ont été sectionnées par filage en fragments de 15 à 150 _ILm de longueur,
• - tous ces fragments ont été alignés selon l'axe du filage,

comme le montrent les figures 1 et 2 représentant des micrographies au grossissement de 130, respectivement d'une coupe du cylindre selon son axe et d'une coupe perpendiculaire à son axe.The bar obtained consists of a cylinder 12 mm in diameter and 3 m in length in which:

• - the carbon fibers were sectioned by spinning into fragments of 15 to 150 _I Lm in length,
• - all these fragments have been aligned along the spinning axis,

as shown in Figures 1 and 2 showing micrographs at 130 magnification, respectively of a section of the cylinder along its axis and a section perpendicular to its axis.

• 1) Les fibres sont toutes alignées suivant une seule direction qui est l'axe du filage. En effet, sur la figure 1, on ne voit en noir que des petits «tirets», bien parallèles, leur longueur étant celle des fibres; sur la figure 2, on ne voit en noir que des «points», leur diamètre étant celui des fibres.
• 2) Les fibres sont très bien dispersées puisque sur la figure 1, il n'y a pas d'amalgames de «tirets», et sur la figure 2, il n'y a pas d'amalgames de «points».

In these figures where the fibers appear in black, it can be seen that:

• 1) The fibers are all aligned in a single direction which is the spinning axis. In fact, in FIG. 1, we see in black only small "dashes", quite parallel, their length being that of the fibers; in FIG. 2, only "dots" are seen in black, their diameter being that of the fibers.
• 2) The fibers are very well dispersed since in FIG. 1, there are no amalgams of "dashes", and in FIG. 2, there are no amalgams of "dots".

The bar is finally cut perpendicular to the axis of the spinning in discs of thickness 1 mm.

The discs obtained were used as covering contacts. It has been found that their performance relative to their cost price is such that they can advantageously replace the silver-cadmium oxide contacts despite a shorter lifespan.

Example 2

This example is a variant of Example 1: copper is replaced by one of its alloys with chromium and zirconium. The percentages by weight are as follows:

The fiber substrate used is the same as in Example 1. The felt is impregnated with the Cu-Cr-Zrfondu alloy, at 1050 ° C., under a pressure of (5 bar) 5.10 + 5 Pa.

The billet obtained contains 84.5% by volume of Cu-Cr-Zr alloy.

This billet is then hot-spun at 820 ° through a die of square section 12 x 12 mm.

The bar obtained consists of a rectangular section 12 x 12 mm and a length of 2.8 m which is cut into slices 1 mm thick which are used as opening contacts.

All the observations indicated in Example 1 are applicable to Example 2.

Example 3

This example is a variant of Example 2.

The Cu-Cr-Zr alloy, after quenching and tempering, sees its mechanical properties markedly increased.

Therefore, after spinning, the bar obtained is heated to 1000 ° C, then quenched with water and annealed at 475 ° C.

It can be seen that the contacts obtained by cutting off this bar have a significantly improved lifetime compared to that of the contacts obtained according to Example 2.

Example 4

This example is a variant of Example 1: copper is replaced by silver. The fiber substrate used is the same as in Example 1. The felt is impregnated with molten silver, at 1000 ° C., under a pressure of (5 bars) 5.10 + 5 Pa.

The billet obtained contains 83% by volume of silver.

This billet is then hot-spun at 780 ° C in the same die. The bar obtained consists of a cylinder with a diameter of 12 mm and a length of 3 m which is cut into discs 1 mm thick which are used as contacts.

It can be seen that these contacts have significantly improved performance compared to those of traditional silver-carbon contacts.

The examples above only illustrate the use of a few metals or alloys. It is clear that other metals and alloys can be used without departing from the scope of the invention: it then suffices to adapt the impregnation and spinning conditions to their physical characteristics.

Claims (4)

1. A process for the production of an electrical contact formed by a substrate of carbon fibres of small diameter which are aligned in a preferential direction and impregnated with a metal or metal alloy which is a good conductor of electric current, characterised in that the carbon fibres forming the substrate are between 2 and 50 _ILm in diameter and that after impregnation under pressure with a metal or alloy which does not react with the carbon, the material produced in that way is subjected to a hot extrusion operation.

2. A process according to claim 1 characterised in that the metal or metal alloy is selected from copper and alloys thereof, silver and alloys thereof and cadmium.

3. A process according to claim 1 or claim 2 characterised in that the carbon fibre substrate is formed by fibres in a loose random arrangement or by a felt of fibres.

4. A process according to any one of claims 1 to 3 characterised in that the carbon fibres of the substrate are of an initial length of a few millimetres to a few centimetres and are cut into fragments measuring 15 to 150 µm by the extrusion operation.

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