Electrical contact compositions and novel manufacturing method

Abstract: A new class of low chop contact materials based on Ag-chromium carbide and Ag-Cr compositions have an essentially 100% dense, porosity free microstructure. These materials combine the advantageous properties of Ag-WC and Cu-Cr contacts without their disadvantages. A method of making this new class of low chop contact materials includes the steps of cold pressing a mixture of Ag and chromium or chromium carbide to form an unsintered blank and the elevated temperature infiltration of silver into the unsintered blank to obtain an essentially 100% dense, porosity free microstructure.

Description: This invention relates generally to making electrical contacts for use in vacuum interrupters that are used for power interruption and control devices, The basic contact and its arrangement in a vacuum interrupter, upon which this invention is an improvement, are well known in the art. The contact material is critical to the successful operation of the vacuum interrupter. As the contacts separate, an electric arc is formed between the contacts. This arc, called a vacuum arc, bums in metal vapor evaporated from the contacts themselves at the arc roots. In an alternating current (ac) circuit where the current follows a sinusoidal wave form to a natural current zero, the energy deposited at the contacts decreases as the current decreases. With a reduction in the energy input to the contact there is a corresponding reduction in the evaporation of the contact material needed to sustain the vacuum arc. A critical property of contact materials used in vacuum interrupters is the current at which there is no longer enough metal vapor to sustain the vacuum arc and it spontaneously extinguishes before the natural current zero. This current is called the "chop current". If the chop current has a high value, the resultant high rate of change of current can cause high voltages in the rest of the circuit. This is especially true if the circuit contains a highly inductive load such as an electric motor. Contact compositions have been developed to produce low chop currents in vacuum interrupters to be used in inductive circuits such as motor circuits. Two well-known contact materials are Ag-WC and the preferred high current, vacuum interrupter, contact material Cu-Cr, containing a small percentage of Bi. Each of these materials relies on a higher vapor pressure material. For example, Ag in the Ag-WC system and Bi in the Cu-Cr-Bi system to provide enough metal vapor for the arc to burn to very low values of current, for example, the order of 1A or less. Both of these contact materials have major disadvantages. The Ag-WC materials interrupt currents lower than about 3500A to 4000A very reliably. However, at higher current the heating of the WC causes it to become a thermonic emitter of electrons and its current interruption performance decreases rapidly as the current is increased. The Cu-Cr-Bi material operates well at high currents. Unfortunately, when large percentages of Bi are used, the reactivity of Bi vapor with other materials results in manufacturing difficulties especially in the high temperature vacuum furnaces used to manufacture the complete vacuum interrupters. Bi vapor can react with and destroy the braze materials used to seal the vacuum interrupters, and they can even destroy the furnace metal windings and vacuum furnace linings. In accordance with the invention, there is now provided a method of making an electrical contact as defined in claim 1. The method according to the invention, is to be contrasted with the three-step procedure according to JP-A-62077439 in which a mixture obtained by blending a refractory material (WC, MoC, Cr 3 C 2 , TiC, W, Mo, Cr, Ti) and an auxiliary (Co, Fe, Ni) with a high conductivity material (Ag and/or Cu) is first compacted then sintered in H 2 and finally infiltrated with additional conductivity material. An effective amount of a ternary element selected from bismuth, tellurium and thallium can also be added to the alloy if required to enhance the arc sustaining vapor. The desired electrical composition is formed by adding 0.10 to 0.99% by weight of the ternary element to the alloy. An effective amount of cobalt may also be added to the desired electrical composition to improve its wetting properties and enhance its essentially 100% dense, porosity free microstructure. The effective amount of cobalt is 0.5 to 2.5% by weight. The alloy suitably comprises 50 to 60% by weight silver and 40 to 50% by weight Cr 3 C 2 or 50 to 60% by weight silver and 40 to 50% by weight Cr. The contact has an essentially 100% dense, porosity free microstructure. The use of Ag in the alloy enhances arc vapor due to higher vapor pressure of Ag compared to Cu at a given temperature. The operation of the contact can be accomplished at lower current due to the lower thermal conductivity of chromium carbide. The method of making this contact comprises the two step process of cold pressing an blank and elevated temperature infiltration of silver into the blank to obtain an essentially 100% dense, porosity free microstructure. The blank is formed by blending 50 to 60% by weight silver powder and a powdered material selected from the group consisting of 40 to 50% by weight Cr 3 C 2 , Cr 7 C 3 , Cr 23 C 6 and Cr, treating the blended powder mass with hydrogen to precoat/presinter the blended powder mass, granulating the blended powder mass and passing it through a mesh screen, reblending the blended powder mass and shaping it into solid blanks. The first blending preferably uses a V-shaped blender with an intensifier bar and is carried out for 30 to 50 minutes, preferably 45 minutes. The hydrogen treatment to precoat/presinter the blended powder mass is carried out at 900° to 1100°C for 40 to 55 minutes, preferably at 1000°C for 45 minutes. The granulated powder mass is passed through a screen of 15 to 25 mesh, preferably a 20 mesh screen. The porous blank is, for example, 80 to 85% of the theoretical density for a Ag-Cr 3 C 2 alloy and 87 to 93% of the theoretical density for a Ag-Cr alloy. The silver infiltration takes place in a hydrogen furnace at 1000 to 1200°C for about 30 minutes to 1 1/2 hours, preferably 1100°C for 1 hour. Infiltration with silver produces an essentially 100% dense, porosity free microstructure by diffusion of liquid Ag through the interconnected porosity within the pressed, unsintered blank. The products made according to the invention have the advantages that: they combine the excellent low chop characteristics of Ag-WC and Cu-Cr-Bi but do not possess their disadvantages; they interrupt higher currents that can be used with Cu-Cr-Bi and that can easily be processed in high temperature vacuum or hydrogen furnaces; they facilitate breakage of any welds resulting from arcing between contact surfaces as the contacts close because stresses or forces required to break such welds will be low; they sustain an arc for a longer than the usual time due to the high vapor pressure of silver compared to copper for more efficient current transfer and vacuum interrupter operation; they enable the vacuum operation to be accomplished at lower currents due to the lower thermal conductivity of chromium carbide; and they permit application at both medium and low voltages. A full understanding of the invention can be appreciated from the following detailed description of the invention when read with reference to the accompanying drawing, which is a photomicrograph at 500X magnification of a silver-chromium carbide contact microstructure with its essentially 100% dense, porosity free microstructure. The method of the invention is for making an improved electrical contact material which comprises an alloy of silver and a material selected from chromium carbide and chromium. The chromium carbide is selected from Cr 3 C 2 , Cr 7 C 3 and Cr 23 C 6 . An effective amount of a ternary element selected from bismuth, tellurium and thallium may also be added to the alloy to enhance an arc sustaining vapor. The effective amount is less than 1% by weight and a desired electrical composition may be formed by adding 0.10 to 0.99% by weight of the ternary element to the alloy during the blending process. If the ternary element is kept below 1% by weight, a high temperature vacuum furnace can be used for manufacturing. An effective amount of cobalt may be added during the blending process to the electrical composition to improve its wetting properties and enhance its essentially 100% dense, porosity free microstructure. The effective amount of cobalt is 0.5 to 2.5 % by weight, preferably 1 to 2%. The alloy suitably comprises 50 to 60% by weight silver and 40 to 50% by weight Cr 3 C 2 or Cr, preferably 58% Ag and 42% Cr 3 C 2 , or preferably about 50% Ag and about 50% Cr. The contact has an essentially 100% dense, porosity free microstructure. The use of Ag in the alloy enhances arc vapor due to the higher vapor pressure of Ag compared to Cu. The operation of the contact can be accomplished at lower current due to the lower thermal conductivity of chromium carbide combined with the high vapor pressure of Ag. The arc bums in the metal vapor evaporated from the contacts. A higher vapor pressure material causes evaporation of the metal at lower currents. The low thermal conductivity of the chromium carbide retains heat longer and gives it out slowly to the Ag, allowing for the Ag metal vapor to support the arc. After arcing, the Cr or chromium carbide becomes finely dispersed in the surface and the surface becomes a brittle skin over the original contact structure facilitating breakage of any weld resulting from arcing between contact surfaces. The method of making this contact comprises a two step process of cold pressing a blank and the elevated temperature infiltration of silver into the blank to obtain an essentially 100% dense, porosity free microstructure. The method further comprises blending silver and a material selected from Cr 3 C 2 , Cr 7 C 3 , Cr 23 C 6 and Cr, treating the blend with hydrogen to precoat/presinter a blended powder mass, granulating the blended powder mass and passing it through a mesh screen, reblending the blended powder mass in a V-shape blender and shaping it into solid blanks. The first blending uses an intensifier bar and takes 30 to 50 minutes, preferably 45 minutes. The hydrogen treatment to precoat/presinter the blended powder mass occurs at 900 to 1100°C for 40 to 55 minutes, preferably at 1000°C for 45 minutes. The granulated powder mass is passed through a screen of 15 to 25 mesh. The porous blank is 80 to 90% of the theoretical density for a Ag-Cr 3 C 2 alloy and 87 to 93% of the theoretical density for a Ag-Cr alloy. The silver infiltration takes place in a hydrogen furnace for 1000 to 1200°C for 30 minutes to 1 1/2 hours, preferably at 1100°C for 1 hour. Infiltration with silver produces an essentially 100% dense, porosity free microstructure. The following Examples illustrate the invention. Example 1 An improved electrical contact comprising about a nominal 58% by weight silver and 42 % by weight Cr 3 C 2 was made by the following method. 1224 grams of silver powder and 1176 grams of Cr 3 C 2 powder were blended in a V-blender fitted with an intensifier bar for 45 minutes. The blended powder mass was given a hydrogen treatment for 45 minutes at 1000°C to precoat/presinter the powder mass. The powder mass was broken up in a granulator and passed through a 20 mesh screen. The blend was then reblended for a few minutes in a V-blender from which the intensifier bar was removed. Solid, cylindrically shaped blanks were then cold pressed to about 80 to 93% of the theoretical density of the Ag-Cr 3 C 2 composition. The blanks were then infiltrated with silver by placing either a pressed disc of silver powder or solid silver, containing an excess silver volume over that required to fill the porosity in the pressed blank, on top of the blank's flat surface and the assembly was then placed in a hydrogen furnace at 1000°C for one hour. After infiltration with silver, the contacts can be machined to desired size by conventional milling and/or turning in a lathe. Before blending it may be advantageous to add less than about 1% by weight of a ternary element such as bismuth, tellurium or thallium powder to the Ag/Cr x C y powder blend for enhancement of the arc. In order to improve the wetting and density of the contact, it may also be advantageous to add 1 to 2 % by weight of cobalt powder to the Ag/Cr x C y powder blend. Example 2 An improved electrical contact comprising about a nominal 50% by weight silver and 50% by weight Cr was made by the following method. 1000 grams of silver powder and 1000 grams of Cr powder were blended in a V-blender fitted with an intensifier bar for 45 minutes. The blended powder mass was given a hydrogen treatment for 45 minutes at 1000°C to precoat/presinter the powder mass. The powder mass was broken up in a granulator and passed through a 20 mesh screen. The blend was then reblended for a few minutes in a V-blender from which the intensifier bar has been removed. Solid, cylindrically shaped blanks were then cold pressed to about 80 to 93% of the theoretical density of the Ag-Cr composition. The blanks were then infiltrated with silver by placing either a pressed disc of silver powder or solid silver, containing an excess silver volume over that required to fill the porosity in the pressed blank, on top of the blank's flat surface and the assembly was then placed in a hydrogen furnace at 1000°C for one hour. After infiltration with silver, the contacts can be machined to desired size by conventional milling and/or turning in a lathe. Before blending, it may be advantageous to add less than about 1 % by weight of a ternary element in powder form such as bismuth, tellurium or thallium for enhancement of the arc to the Ag/Cr blend. In order to improve the wetting and density of the contact, it may also be advantageous to add 1 to 2% by weight of cobalt powder to the Ag/Cr blend. The drawing shows in a photo-micrograph at 500X magnification of the silver-chromium carbide, Ag - Cr 2 C 3 contact, the microstructure made by silver infiltration of the pressed, unsintered contact. The above means of manufacturing consisting of a cold pressing and elevated temperature infiltration of silver gives an essentially 100% dense, porosity free contact microstructure which allows high current interruption.

Claims: A method of making an electrical contact comprising an alloy of Ag and a material selected from Cr x C y and Cr, the method comprising the steps of (a) blending Ag and a material selected from Cr 3 C 2 , Cr 7 C 3 , Cr 23 C 6 and mixtures thereof, and Cr to form a blend, (b) treating the blend with hydrogen to precoat/presinter a blended powder mass. (c) granulating and passing the blended powder mass through a mesh screen, (d) reblending and shaping the blended powder mass into solid blanks, and (e) cold pressing a mixture of Ag and the material selected from, Cr x C y and Cr to form a blank and infiltrating silver at elevated temperature into the blank to obtain an essentially 100% dense, porosity free microstructure. A method according to claim 1, wherein blending step (a) involves the use of an intensifier bar and is carried out for 30 to 50 minutes. A method according to claim 2, wherein blending is carried out for 45 minutes. A method according to any one of claims 1 to 3, wherein treating step (b) is carried out at 900 to 1100°C for 40 to 55 minutes. A method according to claim 4, wherein treating step (b) is carried out at 1000°C for 45 minutes. A method according to any one of claims 1 to 5, wherein step (c) involves the use of a 15 to 25 mesh screen. A method according to claim 6, wherein the screen has a mesh of about 20. A method according to any one of claims 1 to 7, wherein after cold pressing, the blank is 80 to 85% of the theoretical density for a Ag-Cr 3 C 2 alloy and 87 to 93% of the theoretical density for a Ag-Cr alloy. A method according to any one of claims 1 to 8, wherein the step of infiltrating silver into the blank comprises heating the blank in a hydrogen furnace at 1000 to 1200°C for 30 to 90 minutes. A method according to any one of claims 1 to 9, wherein a ternary element selected from bismuth, tellurium and thallium is added during blending step (a) to enhance an arc sustaining vapor. A method according to claim 10, wherein the ternary element is added in an amount of 0.10 to 0.99% by weight. A method according to any one of claims 1 to 11, wherein cobalt is added during blending step (a) to improve the wetting and density characteristics of the electrical contact. A method according to claim 12, wherein cobalt is added in an amount of 0.5 to 2.5% by weight. A method according to any one of claims 1 to 13, wherein the alloy comprises 50 to 60% by weight Ag and 40 to 50% by weight Cr 3 C 2 . A method according to claim 14, wherein the alloy comprises 58% by weight Ag and 42% by weight Cr 3 C 2 .

Inventors: LOVIC WILLIAM ROBERT, SLADE PAUL GRAHAM, WHITLOW GRAHAM ANTHONY, LOVIC,WILLIAM ROBERT, SLADE,PAUL GRAHAM, WHITLOW, GRAHAM ANTHONY
Applicants: EATON CORP, EATON CORPORATION
Application year: 1995
Main classification: H01H1
IPC codes: C22C5, H01H1, H01H11