Low Gas Content Materials Based on Refractory Metals, Low Gas Content Materials Based on Copper-Chromium
Main Articel: [[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 contact
materials 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 vacuum
switches if their total gas content does not exceed approximately 150 ppm. In
the low gas content W/Cu (VAKURIT) material mostly used in vacuum contactors
the high melting W skeleton is responsible for the high erosion resistance when
combined with the high conductivity copper component which evaporates
already in noticeable amounts at temperatures around 2000 °C.
Since there is almost no solubility of tungsten, tungsten carbide, or molybdenum
in copper or silver the manufacturing of these material is performed powdermetallurgically.
The W, WC, or Mo powders are pressed and sintered and then
infiltrated with low gas content Cu or Ag. The content of the refractory metals is
typically between 60 and 85 wt% ''(Figs. 2.142 and 2.143)''.
By adding approximately 1 wt% antimony the chopping current, i.e. the abrupt
current decline shortly before the natural current-zero, can be improved for
W/Cu (VAKURIT) materials ''(Table 2.43)''.
The contact components mostly used in vacuum contactors are usually shaped
as round discs. These are then attached by brazing in a vacuum environment to
their contact carriers ''(Table 2.44)''.
===Low Gas Content Materials Based on Copper-Chromium===
As contact materials in vacuum interupters in medium voltage devices low gas
materials based on Cu/Cr have gained broad acceptance. The typical chromium
contents are between 25 and 55 wt% ''(Figs. 2.144 and 2.145)''. During the
powder metallurgical manufacturing a mix of chromium and copper powders is
pressed into discs and subsequently sintering in a reducing atmosphere or
vacuum below the melting point of copper. This step is followed by cold or hot
re-pressing. Depending on the composition the Cu/Cr (VAKURIT) materials
combine a relatively high electrical and thermal conductivity with high dielectric
stability. They exhibit a low arc erosion rate and good resistance against welding
as well as favorable values of the chopping current in medium voltage load
switches, caused by the combined effects of the two components, copper and
chromium ''(Table 2.43)''.
The switching properties of Cu/Cr (VAKURIT) materials are dependent on the
purity of the Cr metal powders and especially the type and quantity of impurities
contained in the chromium powder used. Besides this the particle size and
distribution of the Cr powder are of high importance. Because of the getter
activity of chromium a higher total gas content of up to about 650 ppm
compared to the limits in refractory based materials can be tolerated in these
Cu/Cr contact materials. Besides the more economical sinter technology also
infiltration and vacuum arc melting are used to manufacture these materials.
Cu/Cr contacts are supplied in the shape of discs or rings which often also
contain slots especially for vacuum load switches in medium voltage devices
''(Table 2.44)''. Increased applications of round discs can also be observed for low
voltage 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
==References==
