RU2228237C1 - Method for making charge of contacts of argentum-graphite compositions - Google Patents

Abstract

FIELD: powder metallurgy, namely manufacture of cermet contacts. SUBSTANCE: method comprises steps of mixing powder containing argentum with powdered graphite in vibration type mill and simultaneously performing additional grinding of above mentioned powders; heating prepared mixture in reducing hydrogen atmosphere until making pelletized charge; using as powder containing argentum powder of argentum carbonate or mixture of powders of argentum and argentum carbonate at content of argentum carbonate no less than 30 mass% on conversion to argentum; performing grinding at mixing process in vibration mill until linear size of particles of argentum carbonate no more than 1 micrometer; heating prepared mixture at temperature 450 - 550 C. EFFECT: enhanced resistance of contacts against sealing of contacts. 1 tbl, 1 ex

Description

The invention relates to powder metallurgy, in particular to the manufacture of ceramic-metal electrical contacts designed to operate in low-voltage circuit breakers to protect electrical circuits from overcurrents.

A known method of manufacturing a charge for electrical contacts from a silver-graphite composition, which consists in the fact that silver powder (PSr1 according to TU 48-1-702-87) and graphite powder (dry colloidal preparation according to TU 113-08-48-63-90 ) are mixed, and then heated in a reducing atmosphere (for example, a hydrogen atmosphere) at a temperature of 400-550 ° C to clump the powder into granules in order to give it the necessary fluidity [1]. The disadvantage of this technical solution is the low resistance to welding at high currents of contacts made of silver-graphite mixture.

There is also known a method of manufacturing a charge for electrical contacts based on silver containing graphite, which consists in the fact that a mixture containing silver and graphite is additionally introduced with a water-soluble nickel salt in an amount of from 0.5 to 5.0 wt.% In terms of nickel and before drying it is granulated [2]. A water-soluble nickel salt is introduced into the mixture in order to improve the quality of granulation and improve the fluidity of the mixture, which must be poured into the molds before pressing the contact blanks. However, the presence of nickel adversely affects the resistance of the contacts to welding in the switching mode of short circuit currents.

Closest to the claimed technical essence is a method of producing a mixture containing silver and graphite, disclosed in the description of the copyright certificate for “Ceramic bimetal electric contact” [3]. The described method consists in the fact that the charge for the material of the working contact layer, containing along with silver 5-35 wt.% Nickel and 2-3.5 wt.% Graphite, is made by mixing a powder containing silver with graphite powder in a vibrating mill with additional grinding, and heating the resulting mixture in a reducing atmosphere of hydrogen is carried out to obtain a granular charge. The disadvantage of this technical solution is the low resistance to welding of contacts made from the silver-graphite mixture obtained in this way, including due to the high nickel content in it.

The objective of the present invention is to increase the resistance to welding of contacts made of silver-graphite mixture.

The inventive method for the manufacture of a charge for contacts from silver-graphite compositions is that a powder containing silver is mixed with graphite powder in a vibratory mill and this mixture is heated to obtain a granular charge in a hydrogen atmosphere at a temperature of 450-550 ° C. The powder containing silver is a powder of silver carbon or a mixture of powders of silver and carbon dioxide, however, in the latter case, the content of carbon dioxide should be at least 30 wt.% In terms of silver. Mixing a mixture of powder containing silver and graphite powder in a vibratory mill should lead to additional grinding of carbon dioxide particles to a linear size of not more than 1 μm.

Due to the use of this method, a brittle powder of silver carbonate salt, crushed to a size of not more than 1 μm, after decomposition when heated in a reducing atmosphere of hydrogen, provides a finely dispersed silver-graphite mixture in which graphite flakes are evenly distributed between small silver particles. Contacts made of this charge by known methods of powder metallurgy have high resistance to welding at high currents transmitted through them.

Description of a specific example of the implementation of the proposed method.

Powders of graphite (TU 113-08-48-63-90) and carbon dioxide (TU 48-1-701-86) or powders of graphite, carbon dioxide silver and silver (PSr1 according to TU 48-1-702-87) are mixed. In the latter case, silver carbon dioxide in the charge should be 30 or more mass percent in terms of silver. Mixing is carried out in a vibratory mill with steel balls for 20-90 minutes until the particle size of carbon dioxide silver is not more than 1 μm. Then the mixture is poured into pallets made of stainless steel or nickel and placed in an electric furnace, where carbon dioxide is decomposed to pure silver in a hydrogen atmosphere at a temperature of 450-550 ° C for one hour. The crushed silver carbon dioxide powder after decomposition provides a finely dispersed mixture in which graphite flakes are evenly distributed between small particles of silver. Then this crumpled mixture is wiped through a sieve to obtain granules and to give the mixture the necessary fluidity. After this, the mixture is ready for the manufacture of contacts by known methods of powder metallurgy (pressing with a specific pressure of 2.0-2.5 t / cm ² ; sintering of pressed billets in a hydrogen atmosphere at 850-880 ° C for 1.5 hours; pressing with specific pressure of 8-10 t / cm ² and annealing to relieve stresses at 400-450 ° C for 30 min).

The resistance to welding of contacts can be estimated by the number of welding of contacts to each other, requiring large breaking forces after passing a large pulse current through them. The greater the number of such welds, the lower the resistance of the contacts to welding.

To confirm the technical effect of the proposed method, 6 compositions of the powder mixture were made, each of which was mixed for a certain time in a certain way and subjected to reductive heating at specific temperatures (see table, columns 1-5). Then, contacts were made from the obtained blends and tested at the SPKV stand according to RD-1-01 of 03/05/01.

Test Modes:

Switching current - 500 A.

The duration of the transmitted current is 3 ms.

Polarity reversal of contacts - through each cycle “on - off”.

The total number of cycles is 100.

The resistance of the contacts to welding is evaluated by the number of welds of contacts to each other, requiring tensile forces greater than 6 N.

The test contact of the silver-graphite composition was used as a fixed contact, and contact from the silver composition was used as a movable contact — 30 wt.% Nickel (as is customary in circuit breakers).

The test was subjected to at least 20 contacts made of a particular mixture. The significance of differences in the arithmetic mean values of the measurement results was estimated by the t-criterion for α = 0.05 [4]. The test results are shown in the 6th column of the table.

The test results show that the resistance to welding of contacts made from a mixture prepared by the inventive method is 3.4-4.2 times higher than the resistance to welding of contacts made from a mixture obtained by the prototype method. The difference between the arithmetic mean values of the number of welds is significant: the resistance index according to example 6 is significantly worse compared to examples 1-5.

It is also obvious that a decrease in the carbon dioxide silver content to a value less than 30 wt.% In terms of silver leads to a decrease in welding resistance: this indicator in Example 4 is slightly worse than the indicators in Examples 1-3.

The increase in the average particle size of silver carbon dioxide in the mixture to 5 μm, which is provided by reducing the mixing time in the vibrating mill to 5 minutes, also leads to a decrease in resistance to welding: this indicator in example 5 is slightly worse than the indicators in examples 1-3.

Thus, the inventive method of manufacturing a charge for contacts from silver-graphite compositions allows the manufacture of contacts by known methods of powder metallurgy, which are highly resistant to welding at high pulsed currents passed through them.

Literature

1. Overview “Powder contact materials of low-voltage equipment”, Moscow, Informelectro, 1986, 64 p.

2. USSR copyright certificate No. 1577173.

3. USSR copyright certificate No. 1313244.

4. Storm R. Mathematical statistics. M.: Mir, 1970, 368 p.

Claims (1)

A method of manufacturing a charge for contacts from silver-graphite compositions, comprising mixing a powder containing silver with graphite powder in a vibratory mill and further grinding, heating the mixture in a reducing atmosphere of hydrogen to obtain a granular charge, characterized in that as a powder containing silver, use carbon dioxide silver powder or a mixture of silver and carbon dioxide powders with a carbon dioxide content of at least 30 wt.% in terms of silver, grinding and mixing is carried out in a vibrating mill to a linear carbon size silver particles less than 1 micron, and heating the resulting mixture is carried out at a temperature of 450 - 550 ° C.