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===Tungsten and Molybdenum (Pure Metals)===
Tungsten is characterized by its advantageous properties of high melting and boiling points, sufficient electrical and thermal conductivity and high hardness and density ''(<xr id="tab:Mechanical_Properties_of_Tungsten_and_Molybdenum"/><!--(Table 2.35)''-->). It is mainly used in the form of brazed contact tips for switching duties , that require a rapid switching sequence , such as horn contacts for cars and trucks.
Molybdenum has a much lesser importance as a contact material since it is less resistant against oxidation than tungsten.
Both metals are however used in large amounts as components in composite materials with silver and copper.
<figtable id="tab:Mechanical_Properties_of_Tungsten_and_Molybdenum"><caption>'''<!--Table 2.35: -->Mechanical Properties of Tungsten and Molybdenum'''</caption><table border="1" cellspacingclass="0" style="border-collapse:collapsetwocolortable"><tr><tdth><p class="s12">Material</p></tdth><tdth><p class="s12">Micro Structure Condition</p></tdth><tdth><p class="s12">Vickers</p><p class="s12">Hardness HV 10</p></tdth><tdth><p class="s12">Tensile Strength</p><p class="s12">[MPa]</p></tdth></tr><tr><td><p class="s12">WolframTungsten</p></td><td><p class="s12">Lightly worked structure</p><p class="s12">(wire and strip > 1.0 mm thick)</p><p class="s12">Strongly worked structure</p><p class="s12">(wire and strip < 1.0 mm thick)</p><p class="s12">Re-crystallized structure</p></td><td><p class="s12">300 - 500</p><p class="s12">500 - 750</p><p class="s12">360</p></td><td><p class="s12">1000 - 1800</p><p class="s12">1500 - 5000</p><p class="s12">1000 - 1200</p></td></tr><tr><td><p class="s12">MolybdänMolybdenum</p></td><td><p class="s12">Lightly worked structure</p><p class="s12">(wire and strip > 1.0 mm thick)</p><p class="s12">Strongly worked structure</p><p class="s12">(wire and strip < 1.0 mm thick)</p><p class="s12">Re-crystallized structure</p></td><td><p class="s12">140 - 320</p><p class="s12">260 - 550</p><p class="s12">140 - 160</p></td><td><p class="s12">600 - 1100</p><p class="s12">800 - 2500</p><p class="s12">600 - 900</p></td></tr></table></figtable>
=== Silver–Tungsten (SIWODUR) Materials===Ag/W (SIWODUR) contact materials combine the high electrical and thermal conductivity of silver with the high arc erosion resistance of the high meltingtungsten metal ''(<xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(Table 2.36)''-->). The manufacturing of materials with typically 50-80 wt% tungsten is performed by the powder metallurgical processes ofliquid phase sintering or by infiltration. Particle size and shape of the starting powders are determining the micro structure and the contact specific properties of this material group ''(Figs<xr id="fig:Micro structure of Ag W 25 75"/><!--(Fig. 2.134 and )-->, <xr id="fig:Micro structure of Ag WC 50 50"/><!--(Fig. 2.135) (Table 2.37--> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/>)''.
During repeated switching under arcing loads , tungsten oxides and mixed oxides (silver tungstates – Ag<sub>2</sub> WO<sub>4</sub> ) are formed on the Ag/W surface , creating 2 4 poorly conducting layers which increase the contact resistance and by this the temperature rise during current carrying. Because of this fact the Ag/W is paired in many applications with Ag/C or Ag/WC/C contact parts.
Silver–tungsten contact tips are used in a variety of shapes and are produced for the ease of attachment with a fine silver backing layer and quite often an additional thin layer of a brazing alloy. The attachment to contact carriers is usually done by brazing, but also by direct resistance welding for smaller tips.
Ag/W materials are mostly used as the arcing contacts in disconnect switches for higher loads and as the main contacts in small and medium duty power
switches and industrial circuit breakers ''(<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/><!--(Table 2.38)''-->). In north and south america they are also used in large volumes in miniature circuit breakers of small to medium current ratings in domestic wiring as well as for commercial power distribution.
<figtable id=== Silver–Tungsten Carbide (SIWODUR C) "tab:Physical Properties of Contact Materials===This group of contact materials contains the typically 40Based on Silver-65 wtTungsten, Silver-% of the very hard Tungsten Carbide and erosion wear resistant tungsten carbide and the high conductivity silver Silver Molybdenum"><caption>'''(Fig. 2.135) (<!--Table 2.36)'7:-->Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum'. Compared to Ag/W the Ag/WC (SIWODUR C) materials exhibit a higher resistance against contact welding ''(Table 2.37)''. The rise in contact resistance experienced with Ag</W is less pronounced in Ag/WC because during arcing a protective gas layer of CO is formed which limits the reaction of oxygen on the contact surface and therefore the formation of metal oxides.caption>
=== Silver–Molybdenum Materials===Ag/Mo materials with typically 50-70 wt% molybdenum are usually produced by the powder metallurgical infiltration process (<xr id="fig:Micro structure of Ag Mo 35 65"/><!--(Fig. 2.135137)--> and <xr id="tab:Physical Properties of Contact Materials Based on Silver-Tungsten, Silver-Tungsten Carbide and Silver Molybdenum"/><!--(Table 2.36)-->). Their contact properties are similar to those of Ag/W materials (<xr id="tab: Micro structure Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/><!--(Table 2.37)-->). Since the molybdenum oxide is thermally less stable than tungsten oxide, the self-cleaning effect of Ag/Mo contact surface during arcing is more pronounced and the contact resistance remains lower than that of Ag/W. The arc erosion resistance of Ag/WC 50Mo however is lower than the one for Ag/W materials. The main applications for Ag/Mo contacts are in equipment protecting switching devices (<xr id="tab:Contact and Switching Properties of Contact Materials Based on Silver – Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)"/50><!--(Table 2.38)-->).
<figure id="fig:Micro structure of Ag W 25 75">
[[File:Micro structure of Ag W 25 75.jpg|left|thumb|<caption>Micro structure of Ag/W 25/75</caption>]]
</figure>
<figure id="fig:Micro structure of Ag WC 50 50">
[[File:Micro structure of Ag WC 50 50.jpg|left|thumb|<caption>Micro structure of Ag/WC 50/50</caption>]]
</figure>
<figure id="fig:Micro structure of -Ag WC 27 C3">
[[File:Micro structure of -Ag WC 27 C3.jpg|left|thumb|<caption>Micro structure of Ag/WC27/C3</caption>]]
</figure>
<figure id="fig:Micro structure of Ag Mo 35 65">
[[File:Micro structure of Ag Mo 35 65.jpg|left|thumb|<caption>Micro structure of Ag/Mo 35/65</caption>]]
</figure>
</div>
<div class="clear"></div>
<figtable id="tab:Physical properties of copper-tungsten materials"><caption>'''Table 2.38: Application Examples and Forms Physical properties of Supply for Contact Materials Basedon Silver–Tungsten (SIWODUR), Silver–Tungsten Carbide (SIWODUR C) and Silver Molybdenum (SILMODUR)copper-tungsten materials'''</caption>
{| class="twocolortable" style== Copper–Tungsten (CUWODUR) Materials==="text-align: left; font-size: 12px"Copper–tungsten (CUWODUR) materials with typically 50|-85 !Material!Tungsten<br/>Content<br/>[wt.% tungsten are produced by the infiltration process with the tungsten particle size selected]!Density<br/>[g/cm<sup>3</sup>]!Melting Point<br/>[°C]!Electrical<br/>Resistivity<br/>[µΩ*cm]!Electrical<br/>Conductivity<br/>[% IACS]!Electrical<br/>Conductivity<br/>[MS/m]!Vickers<br/>Hardness<br/>[HV10]|-|W/Cu 60/40 <br/>|57 - 63|12,9 - 13,3|1083|3,85 - 4,55|38 - 45|22 - 26|150 - 200|-|W/Cu 65/35|63 - 67|13,6 - 14,0|1083|4,17 - 5,0|34 - 41|20 - 24|160 - 210|-|W/Cu 70/30 |68 - 72|13,9 - 14,4|1083|3,85 - 5,56|31 - 38|18 - 22|160 - 230|-|W/Cu 75/25 |73 - 77according to the end application ''(Figs. |14,6 - 15,2.138 – 2.141) (Table 2.39)''. To increase the wettability of the tungsten skeleton by copper a small amount of nickel |1083|4,76 - 5,88|29 - 36|17 - 21|180 - 210|-|W/Cu 80/20|78 - 82|15,3 - 15,9|1083|5,0 - 6,25|28 - 34|16 - 20|180 - 280|-|}< 1 wt% is added to the starting powder mix./figtable>
W/Cu materials exhibit a very high arc erosion resistance ''(Table 2.40)''. Compared to silver–tungsten materials , they are however less suitable to carrypermanent current.
With a solid tungsten skeleton , as it is the case for W/C infiltrated materials with 70-85 wt% tungsten , the lower melting component copper melts and vaporizes in the intense electrical arc. At the boiling point of copper (2567°C) , the still solid tungsten is efficiently “cooled” and remains pretty much unchanged.
During very high thermal stress on the W/Cu contacts, for example during short circuit currents > 40 kA , the tungsten skeleton requires special high mechanical strength. For such applications , a high temperature sintering of tungsten from selected particle size powder is applied before the usual infiltration with copper (example: CUWODUR H).
For high voltage load switches , the most advantageous contact system consists of a contact tulip and a contact rod. Both contact assemblies are usually made usually from the mechanically strong and high conductive CuCrZr material and W/Cu as the arcing tips. The thermally and mechanically highly stressed attachment between the two components is often achieved by utilizing electron beam welding or capacitor discharge percussion welding. Other attachment methods include brazing and cast-on of copper , followed by cold forming steps to increase hardness and strength.
The main application areas for CUWODUR W/Cu materials are as arcing contacts in load and high power switching , in medium and high voltage switchgear as wellas electrodes for spark gaps and over voltage arresters ''(Table 2.41)''.
<figure id="fig:Micro structure of W Cu 70 30 F">[[File:Physical Properties Micro structure of Copper Tungsten CUWODUR Contact MaterialsW Cu 70 30 F.jpg|rightleft|thumb|Physical Properties <caption>Micro structure of Copper Tungsten W/Cu 70/30 (CUWODURfine) Contact Materials</caption>]]</figure>
==References==
[[Contact Materials for Electrical Engineering#References|References]]
[[de:Werkstoffe_auf_Wolfram-_und_Molybdän-Basis]]