Method Of Producing A Two Layer Contact Piece

Abstract

The invention relates to a method of producing a two layer contact piece for high vacuum power switches. An auxiliary metal and a diffusion metal are alloyed by diffusion into a surface of a metallic original body of high electrical conductivity. The auxiliary metal forms a molten phase with the diffusion metal and the metallic original body. The volume of auxiliary metal provides a defined diffusion depth in the original body, at predetermined diffusion conditions.

Description

The present invention relates to a method of producing a two layer contact piece for high vacuum power switches.

Contact materials for high power switches are required to have extremely low amounts of gas, small chopping effects, small welding power and low contact resistance. Also needed is a low burn-off. The chopping effect causes the arc to break or be interrupted during the switching of low currents, whereby voltage peaks occur as a result of the inductivity effect and may result in breakthroughs. To minimize the chopping effect, a small amount of a metal with high vapor pressure is added to the contact material, which reduces the constriction of the arc caused by current forces. Bonded materials with a high melting metal skeleton structure, such as tungsten, molybdenum or rhenium are usable up to a limit, as contact materials in vacuum power switches. Due to the high atom weight of these materials, the electrical voltage stability of the switch, based on the diffusion of metal vapor out of the contact gap, is not restored quickly enought. The switching currents are limited to about 4 kA. Metals having an atomic weight < 65, as for example, copper,iron, cobalt, nickel and beryllium, may be considered for use as original metals that yield an action supplement of high vapor pressure, as an antichopping component. Thus, it is known to use copper contact pieces, which are provided with recesses whereinto the copper bismuth rings are inserted. Alloys with the original metal copper and the antichopping supplement, such as, e.g., bismuth, makes it difficult, at the required low gas content, to distribute the bismuth uniformly and in a defined manner, in the contact layer of the copper. Moreover, when a homogenous copper bismuth alloy is used, it is hard to produce a firm connection, which is resistant to temperature changes, between the contact piece and the carrier metal, since the active component, e.g., bismuth, leads to a considerable brittleness of the contact material and the solder layer and to a reduction of the stability.

The object of the present invention is to overcome heretofore encountered difficulties during the production of two layer contact pieces.

To this end and in accordance with the invention, an auxiliary metal and a diffusion metal is alloyed, through diffusion, into a surface of a metallic original body of high electrical conductivity, whereby the auxiliary metal forms a molten phase with the diffusion metal and the metallic original body.

In carrying out the method, the auxiliary metal and the diffusion metal are brought into contact with a surface of the metallic body as an alloy or a power mixture of specified composition, as a loose powder or a pulverulent pressed mass. The alloy, the powder mixture or the pulverulent pressed mass are subsequently alloyed into the metallic original body by means of diffusion, so that the amount of the auxiliary metal provides a defined diffusion depth in the original body at predetermined diffusion conditions.

It is possible to use copper, nickel, iron, cobalt or beryllium, as the original body, and bismuth, lead, tellurium or antimony, for example, as the diffusion metal.

It is preferred to use such metals, as auxiliary metals, which form a liquid phase with said metallic original body and with the diffusion metal, at least 50.degree.C below the melting temperature of the metallic original body.

The use of an auxiliary metal offers the advantage that a diffusion temperature may be used which is considerably below the melting temperature of the metallic original body. When the original body consists of copper, the auxiliary metal of silver and the diffusion metal of bismuth, a diffusion temperature between 800.degree. and 1,000.degree. C is suitable in order to obtain, within a period of 10 to 30 minutes, a state of equilibrium and to produce thereby the desired diffusion layer. When pure bismuth is used as a diffusion metal, and copper is employed as the original body, a desired equilibrium Bi content of 2 % would result at a diffusion temperature of 1,075.degree. C, without the use of an auxiliary metal. Since this temperature is only 8.degree. C below the melting temperature of the copper, it is virtually impossible to comply with the required temperature in a furnace for the purpose of manufacture.

The invention will be shown in greater detail with reference to the Drawing, in which there is shown in FIGS. 1 to 5, the steps of carrying out the invention.

A wafer-shaped contact specimen (original body) 11 of 60 mm diameter and a height of 20 mm, is cast from a highly degassed copper, is illustrated in FIG. 1.

In FIG. 2, an annular recess 12 with dimensions .phi..sub.a = 40 mm, .phi..sub.i = 30 mm, depth = 5 mm, and corresponds approximately to the future contact area, is provided in a surface of the original body. A pressed mass of powder 13 (FIG. 3) comprising a mixture of 10 g silver, 15 g copper and 1 g bismuth, and adjusted to a diffusion temperature of 1,000.degree. C is inserted into the recess 12. This produces a liquid phase, forming an equilibrium of 10 g silver and approximately 40 g copper, which contains a uniform distribution of the bismuth. The liquid phase which corresponds to the diffusion range 14, is shown in FIG. 4. After the diffusion is completed, the contact piece which consists of the carrier layer 11 and the indiffused contact area 15, is produced as shown in FIG. 5. The diffusion zone boundary of the contact area 15 is illustrated by the dashed lines. The contact piece, thus obtained, may be easily connected by eutectic means with the carrier metal copper, through customary solders such as for example, AgCu.

When iron, nickel, cobalt or beryllium are used for the original body, appropriate auxiliary metals are to be selected which form at diffusion temperature a liquid phase with the metal of the original body that melts at lower temperatures than the metal of the original body. The liquid phase produces, in the form of mixed crystals, an eutectic or peritectic, depending on the solubility rate, and the amount of auxiliary metal and diffusion metal is determined for a desired depth of penetration.

When using Cu as the original metal, the auxiliary metal may be selected from Ag, Cd, Ge, In, Mg, Si, Sn, Zn, Ce and the diffusion metal may be selected from Te, Bi, Pb, and Sb.

When using Fe as the original metal the auxiliary metal may be selected from Cu, Be, Ce, Fe+(1-4.5) wt-% C, Ge, Nb, Sb, Si, Ti and the diffusion metal may be selected from Te, Bi and Pb.

When using Co as the original metal the auxiliary metal may be selected from B, Co+I1-3) wt-% C, Ge, Nb, Sb, Si, Sn, Ti and the diffusion metal may be selected from Te, Bi and Pb.

When using Ni as the original metal, the auxiliary metal may be selected from B, Be, Ni+(1-2.5) wt-% C, Ce, Mg, Nb, Sb, Si, Sn and the diffusion metal may be selected from Te, Bi or Pb.

When using Be as the original metal, the auxiliary metal may be selected from Ag, Cu, Si and the diffusion metal may be selected from Te, Bi or Pb.

Claims

I claim:

1. A process for preparing a two layer contact piece for high vacuum power switches which comprises alloying a diffusion metal and an auxiliary metal into a surface of a metallic original body of high electrical conductivity, thereby forming a molten phase with the diffusion metal and the metallic original body, said auxiliary metal being a metal which forms a liquid phase with the metallic original body and with the diffusion metal at a temperature at least 50.degree. C below the melting point of the metallic original body.

2. The process of claim 1, wherein the diffusion metal and the auxiliary metal are together brought into contact with the surface of the metallic original body and then diffused into the original body, the amount of the auxiliary metal providing a defined diffusion depth in the original body at the diffusion conditions.

3. The process of claim 2, wherein a body constituting the diffusion metal and the auxiliary metal are brought into an annular recess in the surface of the original body.

4. The process of claim 3, wherein a metal selected from copper, nickel, iron, cobalt and beryllium, is used as the original body.

5. The process of claim 3, wherein a metal selected from bismuth, lead, tellurim and antimony, is used as the diffusion metal.

6. The process of claim 3, wherein a metal selected from copper, nickel, iron, cobalt and beryllium is used as the original body, a metal selected from bismuth, lead, tellurim and antimony is used as the diffusion metal, a metal which forms a liquid phase with the metallic original body and with the diffusion metal at a temperature at least 50.degree. C below the melting point of the original metal, is used as the auxiliary metal.