FRIALIT-DEGUSSIT Technical Ceramics

 

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Ceramic-metal Seals

Design Considerations

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

Operating Voltage
Current-carrying Capacity
Electrical Resistance
Bending and Compressive Strength
Metallization and Brazing Techniques
Operating Temperature Range
Design Characteristics
Installation Requirements

Operating Voltage

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.
If the ceramic body shields the electrodes, the flashover voltage depends only on the leakage path; in this case, the operating voltage is given in the graph.


 

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Current-carrying Capacity

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.

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Electrical Resistance

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.

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Bending and Compressive Strength

The compressive strength of FRIALIT is higher than that of metals at any temperature.
The bending strength is lower than that of metals at room temperature, but exceeds it at temperatures above 700C.
This effect is due to the fact that the elasticity limit and the breakage limit coincide in sintered oxide ceramic materials and that any existing stress cannot be relieved by plastic flow processes.  At higher temperatures, the increase in plasticity compensates for the strength decrease due to thermal effects.

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Metallization and Brazing Techniques

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 1200C 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:

Composition ISO designation Brazing Temperature C
Ag-Cu B-Ag72Cu-780 780
Ag-Cu-Pd various 805-1015
Au-Ni B-Au82Ni-950 950
Cu-Ge B-Cu90Ge-900/1000 1000
Au B-Au-1063 1070
Ti-Ag-Cu B-Ag64CuTi-780/810 850-950

Our metallised FRIALIT components can also be electro-tinned before delivery in order to permit the use of soft solders at low temperatures.
The strength of the brazed metal-ceramic connections is more than 50 MPa where the metallisation width is at least 2mm.  With a suitable design, this strength can also be obtained by a special brazing method without metallisation.

We can also produce ceramic-ceramic bonding using a special glass-brazing technique.

The graph below shows a few thermal expansion curves of metals and alloys as a function of temperature compared with FRIALIT.  This shows that the differences in expansion which occur at the high temperatures during the brazing process must lead to mechanical stress in the ceramic material at lower temperatures.  However, it is normally possible, by suitable design principles, to dissipate these stresses or keep them within tolerable limits.

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Operating Temperature Range

Brazed metal-ceramic assemblies are particularly suitable for use at higher temperatures up to 600C.
If used without gold-plating, these feedthroughs can be used to a working temperature of 350C in air, this being extended to 450C in the case of gold-plated versions.
Other electroplated surface layers such as Cu, Ni, or Ag can also be applied.
Selection of other metal parts, such as pure nickel or titanium can also improve the corrosion resistance and operating temperature range.
Most of the brazed metal-ceramic assemblies can be used at temperatures down as far as -270C.
The thermal shock resistance of the feedthroughs depends on the size and shape of the parts and is approximately 180C (MIL, Standard 220 C, Method 107 B, Condition C)

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Design Characteristics

Thermal 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.
The vacuum leak test of metal-ceramic seals is generally tested with mass spectrometers at a differential pressure of 760 Torr, using helium as the test gas.  The sensitivity of this test method is of the order of 10-9 mbar l/sec.
Pressure vessels with pressures up to 40 bar are available for testing the pressure resistance of FRIALIT feedthroughs.
In the case of metal-ceramic seals subjected to high mechanical stresses, care must be taken that only compressive stresses occur within the ceramic material whenever possible.  If this design principle is remembered, extremely strong pressure resistant feedthroughs can be constructed with FRIALIT.

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Installation Requirements

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:

  General name Typical Trade Names Standard Notation Material Number
Fe steel 9 S 20 1.0711
CrNiFe stainless steel V 2 A
V 4 A
x 5 CrNi 18 10
x 6 CrNiTi 18 10
x 6 CrNiMoTi 17 12 2
1.4301
1.4541
1.4571
NiFe 42 Alloy 42 VACODIL 42 Ni 42 1.3917
NiFeCo Alloy K
 
VACON 11
VACON 70
NiCo 29 18
NiCo 28 23
1.3981
1.3982
Cu OF-Cu
SE-Cu
2.0040
2.0070
Ni Ni 99.6
LC-Ni 99
2.4060
2.4068
Ti Contimet 30 3.7024
Mo Pure molybdenum
min 99.95%

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Last modified:
Tuesday, 08 January 2013