FRIALIT®-DEGUSSIT® Technical Ceramics
There are many considerations when designing a brazed metal-to-ceramic assembly. These include the required performance, the application conditions and the manufacturing technology. We will attempt to summarise some of the key elements below
Specification of the flashover voltage values is referred to the
distance between two voltage-carrying electrodes under normal conditions
(760mm Hg, 50% relative humidity) at which no flashover occurs. In
vacuum of approximately 10-6 mbar these values increase 3-4
times. In pressurised suitable dry gases, such an increase can
also be obtained. For voltage values below 10 kV no convolutions
are necessary. Furthermore, the values are depending on the
geometry of the electrodes.
The current-carrying capacity of the feedthroughs depends mainly on the type of metal wire used. the specifications in the graph shown here should be regarded only as approximate values. In specific cases, the dissipation of heat through the insulator must also be taken into account.
If the electrical resistance of various insulating ceramics as a function of temperature are compared, it becomes apparent that FRIALIT ceramics retain their good insulating properties even at higher temperatures. This high electrical resistance (approximately 1015 Ohm-cm at room temperature for F99.7) is the result of the almost complete lack of the alkali ions, whereas porcelain and other ceramic insulating materials contain easily-moved electrical charges. Our feedthroughs and insulating tubes are therefore made of F99.7. To avoid the natural surface of the oxide ceramic becoming dirty during handling or in service, which would decrease the electrical resistance of the surface, we frequently cover it with a high-melting point glaze, which possesses excellent insulating properties.
The compressive strength of FRIALIT is higher than that of metals at any
FRIALIT parts are mainly metallised using the Mo-Mn process. Nickel-plating can be applied to this basic metal coating. This metallisation makes it possible to carry out brazing operations up to temperatures of 1200°C in a hydrogen atmosphere or in a vacuum. The FRIALIT ceramic components prepared with this metallisation process can be brazed to thermally-matched metal components: e.g. NiFe42 (Alloy 42, VACODIL etc.), NiFeCo (VACON, Kovar etc.), Ti, Mo, etc. Brazing must be carried out in a protective atmosphere or in a vacuum, whereby the vapour pressure of the brazing material must be matched to the method in each case. The following brazing alloys can be used:
metal-ceramic assemblies are particularly suitable for use at higher
temperatures up to 600°C.
stresses also occur during welding or brazing when the brazed
metal-ceramic seals made of FRIALIT are installed and these stresses
must be kept away from the ceramic-metal boundaries wherever possible.
This can be taken into account by suitable design of the flanges or by
the use of special heat sinks.
FRIALIT brazed metal-ceramic assemblies can be connected to other metal components by resistance welding, induction brazing under a protective atmosphere, argon-arc welding, copper cold welding, or electron beam welding. In all cases, regardless of the method, care must be taken that excessive temperature gradients do not occur in close proximity to the brazed connection between the metal and the ceramic material. An optimum design exists if further processing of the feedthroughs is included in the design considerations.
Unless otherwise specified, the tolerances for metal parts are DIN 2768 m, and for ceramic according to DIN 40680 m
Definition of symbols used in drawings: