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Pfeiffer <strong>Vacuum</strong><br />
Page 130<br />
<strong>Vacuum</strong> <strong>Technology</strong><br />
In addition, a special geometric design is also required that sometimes differs from the nor-<br />
mal welded connections that are employed for non-vacuum applications. Wherever possible<br />
in terms of engineering, interior welds must be provided in order to avoid vacuum-side gaps<br />
and cracking, so-called latent leaks. If this is not possible, the weld must extend through to<br />
the vacuum side. Where necessary, a supplemental atmosphere-side weld can be employed<br />
to increase mechanical stability.<br />
In this connection, it is important that this supplemental weld not be continuous in order to<br />
allow leak detection, if necessary, and have no air inclusions. In addition to the TIG welding<br />
process, microplasma welding also plays a role in vacuum technology, particularly for welding<br />
extremely thin-walled components and, to an increasing extent, in electron beam welding,<br />
which must be performed under vacuum.<br />
Brazed connections<br />
In addition to welding, the brazing process is also used to join metals. Brazed joints at<br />
soldering temperatures of above 600 °C are used almost exclusively in vacuum technology.<br />
In order to eliminate the need for highly corrosive flux when soldering, which usually involves<br />
high vapor pressure, and in order to obtain oxide-free, high-strength joints, the soldering<br />
process is performed under vacuum or in a clean inert gas atmosphere. Soft solder joints<br />
are not suitable for vacuum applications. They typically cannot be baked out, have less<br />
mechanical strength and in addition to tin frequently contain other alloy components with<br />
high vapor pressures.<br />
Fusing<br />
The fusing process is an alternative that is primarily used for joining glass components<br />
(in glass equipment) and for glass-to-metal connections. Glass-to-metal fusings are especially<br />
important in the production of vacuum-tight current feedthroughs, for bakable sight glasses<br />
and in the production of vacuum gauges. To fuse glass-to-metal transitions, the materials<br />
must be selected in such a manner that the thermal expansion coefficients of these materials<br />
are as similar to one another as possible throughout a broad temperature range. Numerous<br />
special alloys have been developed for this purpose that are known under trade names such<br />
as Fernico, Kovar, Vacon, Nilo, etc. Fusings with quartz glass are difficult to perform, as this<br />
material has an extremely low thermal expansion coefficient; no metal or metal alloy even<br />
comes close.<br />
Metalized connections<br />
Ceramic-to-metal connections are used for highly bakable and highly insulating current feedthroughs.<br />
They are also employed for manufacturing high-performance transmitting tubes and<br />
for configuring ceramic vacuum chambers for particle accelerators at major physics research<br />
facilities. In the case of this connection technology, the ceramic, e.g. aluminum oxide (92 %<br />
to 98 % Al 0 ), is pre-metalized at those points to be joined with the metal. In this connection,<br />
2 3<br />
it is particularly important to ensure that the thin metal layer (molybdenum or titanium) crease<br />
an intensive connection with the ceramic substrate that is free of voids and pores. Applied to<br />
this is a layer of nickel; this enables a metal cap to be brazed on, for example, to which the<br />
current conductors of the current feedthrough are subsequently soldered.<br />
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