Changes

The contact parts are important components in switching devices. They have tomaintain their function from the new state until the end of the functional life of thedevices.

The requirements on contacts are rather broad. Besides typical contact propertiessuch as

they have to exhibit physical, mechanical, and chemical properties like high electricaland thermal conductivity, high hardness, high corrosion resistance, etc and besidesthis should have good mechanical workability, and also be suitable for good weld andbrazing attachment to contact carriers. In addition they must be made fromenvironmentally friendly materials.

Materials suited for use as electrical contacts can be divided into the following groupsbased on their composition and metallurgical structure:

*'''Pure metals''' From this group silver has the greatest importance for switching devices in the higherenergy technology. Other precious metals such as gold and platinum are only used inapplications for the information technology in the form of thin surface layers. As a nonpreciousmetal tungsten is used for some special applications such as for example asautomotive horn contacts. In some rarer cases pure copper is used but mainly pairedto a silver-based contact material. *'''Alloys'''

Besides these few pure metals a larger number They indicate the boundaries of liquid and solid phases and define the parameters of alloy materials made by meltsolidification.technology are available for Alloying allows to improve the use as contacts. An alloy is characterized by properties of one material at the factthat its components are completely or partially soluble in each other in cost of changing them for the solid statesecond material.Phase diagrams for multiple metal compositions show As an example, the number and type hardness of a base metal may be increased while at thecrystal structure as a function of same time the temperature and composition electrical conductivity decreases with even small additions of the second alloying componentscomponent.

*Composite Materialsmaterials are a material group whose properties are of great importance for electrical contacts that are used in switching devices for higherelectrical currents.

Composite materials are a material group whose properties are of greatimportance for electrical contacts that are used in switching devices for higherelectrical currents.Those used in electrical contacts are heterogeneous materials composed of twoor more uniformly dispersed components in which the largest volume portionconsists of a metal.The properties of composite materials are determined mainly independent fromeach other by the properties of their individual components. Therefore it is forexample possible to combine the high melting point and arc erosion resistanceof tungsten with the low melting and good electrical conductivity of copper, orthe high conductivity of silver with the weld resistant metalloid graphite.

Figure 2The properties of composite materials are determined mainly independent from each other by the properties of their individual components.1 Therefore it is for example possible to combine the high melting point and arc erosion resistance of tungsten with the low melting and good electrical conductivity of copper, or the high conductivity of silver with the weld resistant metalloid graphite. <xr id="fig:Powder metallurgical manufacturing of composite materials (schematic)"/> shows the schematic manufacturing processes from powderblending to contact material. Three basic process variations are typically

During ''sintering without a liquid phase'' <figure id="fig:Powder metallurgical manufacturing of composite materials (left side of schematic) the powder mix is">first densified by pressing, then undergoes a heat treatment [[File:Powder metallurgical manufacturing of composite materials (sinteringschematic), andeventually is re-pressed again to further increase the density. The sinteringatmosphere depends on the material components and later application; avacuum is used for example for the low gas content material Cu/Cr. Thisprocess is used for individual contact parts and also termed pressjpg|thumb|<caption>Powder-sinterrepressmetallurgical manufacturing of composite materials (PSRschematic). For materials with high silver content T_s = Melting point of the starting point atpressing is most a larger block (or billetlower melting component) which is then after sintering hotextruded into wire, rod, or strip form. The extrusion further increases the density</caption>]]of these composite materials and contributes to higher arc erosion resistance.Materials such as Ag</Ni, Ag/MeO, and Ag/C are typically produced by thisprocess.figure>

During ''Sintering with sintering without a liquid phase'' has (left side of schematic) the advantage of shorter process times due powder mix is first densified by pressing, then undergoes a heat treatment (sintering), and eventually is re-pressed again tofurther increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the accelerated diffusion low gas content material Cu/Cr. This process is used for individual contact parts and also results in neartermed press-theoretical densities sinterrepress (PSR). For materials with high silver content the starting point at pressing is most a larger block (or billet) which is then after sintering hot extruded into wire, rod, or strip form. The extrusion further increases the density of thethese composite materials and contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO, and Ag/C are typically produced by this process.

Fig. 2.1: Powder''Sintering with liquid phase'' has the advantage of shorter process times due to the accelerated diffusion and also results in near-metallurgical manufacturing of composite materials (schematic)T = Melting point theoretical densities of the lower melting component composite material. To ensure the shape stability during the sintering process it

As opposed to the liquid phase sintering which has limited use for electricalcontact manufacturing, the ''Infiltration process'' as shown on the right side of theschematic has a broad practical range of applications. In this process thepowder of the higher melting component sometimes also as a powder mix witha small amount of the second material is pressed into parts and after sinteringthe porous skeleton is infiltrated with liquid metal of the second material. Thefilling up of the pores happens through capillary forces. This process reachesafter the infiltration near-theoretical density without subsequent pressing and iswidely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts,controlling the amount or excess on the bottom side of the contact of theinfiltration metal Ag results in contact tips that can be easily attached to theircarriers by resistance welding. For larger Cu/W contacts additional machining isoften used to obtain the final shape of the contact component.

Pure Gold is besides Platinum the chemically most stable of all precious metals.In its pure form it is not very suitable for use as a contact material in electromechanical devices because of its tendency to stick and cold-weld at even low contact forces. In addition it is not hard or strong enough to resistmechanical wear and exhibits high materials losses under electrical arcingloads. This limits its use in form of thin electroplated or vacuum deposited layers.

Main ArticelArticle: [[Gold Based Materials| Gold Based Materials]]

The platinum group metals include the elements Pt, Pd, Rh, Ru, Ir, and Os ''[[Platinum_Metal_Based_Materials|Table 1]]<!--(Table2.6)''-->. For electrical contacts platinum and palladium have practical significanceas base alloy materials and ruthenium and iridium are used as alloying components.Pt and Pd have similar corrosion resistance as gold but because of theircatalytical properties they tend to polymerize adsorbed organic vapors on contactsurfaces. During frictional movement between contact surfaces the polymerizedcompounds known as “brown powder” are formed which can lead to significantlyincrease in contact resistance. Therefore Pt and Pd are typically used as alloys andnot in their pure form for electrical contact applications.

Main ArticelArticle: [[Platinum Metal Based Materials| Platinum Metal Based Materials]]

Tungsten and Molybdenum (Pure Metals), Silver–Tungsten (SIWODUR) Materials, Silver–Tungsten Carbide (SIWODUR C) Materials, Silver–Molybdenum (SILMODUR) Materials, Copper–Tungsten (CUWODUR) Materials Main ArticelArticle: [[Tungsten and Molybdenum Based Materials| Tungsten and Molybdenum Based Materials]]

 Main ArticelArticle: [[Special Contact Materials (VAKURIT) for Vacuum Switches| Special Contact Materials (VAKURIT) for Vacuum Switches]] The trade name VAKURIT is assigned to a family of low gas content contactmaterials developed for the use in vacuum switching devices ''(Table 2.42)''. ===Low Gas Content Materials Based on Refractory Metals===Contact materials of W/Cu, W/Ag, WC/Ag, or Mo/Cu can be used in vacuumswitches if their total gas content does not exceed approximately 150 ppm. Inthe low gas content W/Cu (VAKURIT) material mostly used in vacuum contactorsthe high melting W skeleton is responsible for the high erosion resistance whencombined with the high conductivity copper component which evaporatesalready in noticeable amounts at temperatures around 2000 °C. Since there is almost no solubility of tungsten, tungsten carbide, or molybdenumin copper or silver the manufacturing of these material is performed powdermetallurgically.The W, WC, or Mo powders are pressed and sintered and theninfiltrated with low gas content Cu or Ag. The content of the refractory metals istypically between 60 and 85 wt% ''(Figs. 2.142 and 2.143)''. By adding approximately 1 wt% antimony the chopping current, i.e. the abruptcurrent decline shortly before the natural current-zero, can be improved forW/Cu (VAKURIT) materials ''(Table 2.43)''.The contact components mostly used in vacuum contactors are usually shapedas round discs. These are then attached by brazing in a vacuum environment totheir contact carriers ''(Table 2.44)''. ===Low Gas Content Materials Based on Copper-Chromium===As contact materials in vacuum interupters in medium voltage devices low gasmaterials based on Cu/Cr have gained broad acceptance. The typical chromiumcontents are between 25 and 55 wt% ''(Figs. 2.144 and 2.145)''. During thepowder metallurgical manufacturing a mix of chromium and copper powders ispressed into discs and subsequently sintering in a reducing atmosphere orvacuum below the melting point of copper. This step is followed by cold or hotre-pressing. Depending on the composition the Cu/Cr (VAKURIT) materialscombine a relatively high electrical and thermal conductivity with high dielectricstability. They exhibit a low arc erosion rate and good resistance against weldingas well as favorable values of the chopping current in medium voltage loadswitches, caused by the combined effects of the two components, copper andchromium ''(Table 2.43)''. The switching properties of Cu/Cr (VAKURIT) materials are dependent on thepurity of the Cr metal powders and especially the type and quantity of impuritiescontained in the chromium powder used. Besides this the particle size anddistribution of the Cr powder are of high importance. Because of the getteractivity of chromium a higher total gas content of up to about 650 ppmcompared to the limits in refractory based materials can be tolerated in theseCu/Cr contact materials. Besides the more economical sinter technology alsoinfiltration and vacuum arc melting are used to manufacture these materials.Cu/Cr contacts are supplied in the shape of discs or rings which often alsocontain slots especially for vacuum load switches in medium voltage devices''(Table 2.44)''. Increased applications of round discs can also be observed for lowvoltage vacuum contactors. Table 2.42: Physical Properties of the Low Gas Materials (VAKURIT) for Vacuum Switches Fig. 2.142: Micro structure of W/Cu 30Sb1– low gas Fig. 2.143: Micro structure of WC/Ag 50/50– low gas Fig. 2.144: Micro structure of Cu/Cr 75/25– low gas Fig. 2.145: Micro structure of Cu/Cr 50/50– low gas Table 2.43: Contact and Switching Properties of VAKURIT Materials Table 2.44: Application Examples and Form of Supply for VAKURIT Materials