Wave transmission lines and networks – Long line elements and components – Waveguide elements and components
Patent
1990-07-24
1992-08-04
Group, Karl
Wave transmission lines and networks
Long line elements and components
Waveguide elements and components
501127, 501153, 501120, 156108, 264 65, H01P 108
Patent
active
051362724
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a ceramic component having a plurality of improved properties and to a process for the production of such a component.
Power millimeter and centimeter waves (power exceeding a few watts) are used especially in telecommunications and radars, but there is another application in which high powers are required this is the heating of the plasma of a thermonuclear fusion reactor. In such an application, powers exceeding a few megawatts are required.
These high powers are generated in vacuum tubes gyrotron, klystron, magnetron . . . ). Their principle of operation is in all cases the same: an electron gaining (or losing) velocity absorbs (or emits) electromagnetic energy. It is therefore sufficient to locate on the path of an electron beam a magnetic and electric field in a judicious manner in order that an electromagnetic wave should be generated. As this wave is created under vacuum, it is necessary to cause it to pass through a window in order to use it in an electronic circuit.
This window must be both transparent to the electromagnetic waves and sealed against gases (a vacuum of the order of 10.sup.-8 torr must be capable of being withstood steadily for a period of about ten years).
The windows which are currently employed on microwave tubes are made of metallizable alumina sintered under load (H.P.). The expression "metallizable alumina" is understood as referring to a material on which an appropriately braised small plate of metal has a resistance to tearing off exceeding 300 kg/cm.sup.2. It contains 94 to 98% of alumina, the remainder being most frequently oxides of molybdenum, of manganese or of silicon. The grains of this material frequently have the particular feature of being very large (dimensions exceeding 10 microns).
Although the actual mechanism of strengthening of the adhesion is not yet entirely clear, the metallizable aluminas form part, and this is their strength, of the materials exhibiting the greatest values of resistance to tearing off: values as high as one tonne per square centimeter have been obtained.
However, in the aforementioned application, this metallizable alumina exhibits certain disadvantages:
It is in two phases, that is to say that it is composed of alumina grains separated from one another by a vitreous (non-crystalline) phase. These two materials have differing dielectric constants; this may locally deform the electric field lines and induce breakdowns. Furthermore, they have differing coefficients of expansion; the effect of this, in the event of a local increase in temperature, is to generate incipient cracks.
By reason of its poor dielectric losses, the vitreous phase contributes to absorbing a small part of the
The grains are very large (10-50 .mu.m); the result of this is to diminish the mechanical strength as compared with a fine-grain (1 .mu.m) ceramic.
The subject of the present invention is a ceramic component exhibiting a coefficient of expansion and a dielectric constant which are virtually constant in a part of the volume of the component, this component having low dielectric losses and a very good mechanical strength.
The subject of the invention is also a window of the aforementioned type, which may be metallized while still being homogeneous and transparent to the electromagnetic waves, and effecting virtually no modifications to the lines of the field passing through it.
The subject of the invention is likewise a process for the production of such a component and of such a window.
The component according to the invention comprises a metallizable ceramic co-sintered with a material having properties which are homogeneous at the microscopic level, these properties being properties which are physical, especially dielectric, and/or chemical and/or mechanical.
The window according to the invention comprises at least one central part made of fine-grain ceramic, co-sintered with annular parts made of metallizable ceramic.
The production process according to the invention consists in casting, on the one hand, a slip based on ceramic
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Kormann Rene A.
Loiseau Raymond
"Thomson-CSF"
Group Karl
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