DE610407C - Insulating body or layer for indirectly heated cathodes of electrical discharge vessels - Google Patents

Description

Insulating body or layer for indirectly heated cathodes of electrical discharge vessels A common embodiment of indirectly heated cathodes consists of a wire-shaped heating element that is covered with an insulating layer or is inserted in an insulating body, and of a cathode sleeve surrounding the heating body, which carries the emitting substances . For these cathodes to function properly, it is essential that the insulating layers have a high electrical resistance at the operating temperature. The silicate-containing porcelain-like masses that were first used for this purpose did not meet this requirement and caused numerous disturbances in the operation of the tubes. Better results have been achieved with insulating bodies or layers made of pure oxides, such as Mg 0, A12 03, Zr 02, etc., and cathodes equipped with them have generally proven themselves well for tubes with heating voltages of up to about ao volts.

More recently, efforts have been made to use cathodes that can be heated indirectly to operate with high heating voltages. The tubes equipped with it should be in Direct series or parallel connection (i.e. without the interposition of transformers with alternating current or series resistors with direct current) heated from the lighting network will. Experiments have shown that insulating layers made of the known oxides for are unsuitable for this purpose, as they are apparently under the influence of the imaginary of the operation between cathode sleeve and filament (with parallel connection also high voltages occurring between the filament ends of a cathode) suffer, which usually after a short time lead to a breakdown and thus to destruction the tube lead. The best results were still with aluminum oxide as an insulating material achieved. The oxide can, however, because of its great hardness and poor sinterability difficult to process because of. In addition, there is also the relatively low melting point contrary to its general applicability.

It has now been found that certain compounds or similarly behaving mixtures of aluminum oxide have surprisingly high dielectric strengths and, moreover, can be processed much more easily than the pure oxide. These are the compounds of aluminum oxide of the spinel type, such as MgO, A1203, Be0, Ale 03, Ca O, A12 03. The electrical resistance of these compounds at annealing temperatures is sometimes much higher than that of aluminum oxide. Likewise, the melting point for most of these compounds is several hundred degrees above that of aluminum oxide. These substances are now used according to the invention for the production of insulating bodies and layers for cathodes that are to be heated indirectly used. It is advantageous not to start from naturally occurring compounds (which are usually heavily contaminated), but to produce them from the pure oxides by intimate mixing and firing.

Some embodiments of the new method are briefly described below described. Used for covering heating wires or lining cathode sleeves one z. B. a slurry of the powdery compounds in collodion or similar liquids through which one can move the parts to be coated. leads or with where they are sprayed. After drying, the coatings are then applied in the usual way Burned onto the carrier. The powder compounds can also be used without Knead difficulties with tragacanth glue and similar substances into a dough and inject them into insulators of various shapes. The body dr then advantageously burned at about 160 °. Have the insulating body produced in this way excellent strength.

It is also easily possible to use the oxide mixture directly Manufacture of the insulating bodies and layers to be used. The transformation of the mixture In this case, the insulating body enters into the connection during the sintering process or during the sintering of the insulating layers.

If you use mixtures of the powdered Compound and not yet converted oxide. amount used, so you can continue thereby largely affect the pore volume and the grain size of the insulating body. This is important because insulating bodies with a certain pore volume - especially seem to be puncture proof.

Finally, instead of a compound, you can use a mixture from several connections, e.g. B. Mg O A12 03 with B e O A12 03, to produce the Use insulators and layers.

Claims (3)

PATENT CLAIMS: i. Insulating body or layer for indirectly heated Cathodes of electrical discharge vessels, characterized in that these Body or this layer wholly or partly made up of compounds of aluminum oxide of the spinel type, for example of Mg O Al., 03, Be O A12 03, Ca O A1203. 2. Insulating body according to claim i, characterized in that this body or this layer consists of mixtures of several spinels. 3. Method of manufacture of insulating bodies or layers according to claims 1 and 2, characterized in that that first a mixture of oxides corresponding to the composition of the compound, optionally with the addition of a binder, brought into the desired shape and then the transformation of the mixture into the compound by annealing treatment he follows.