Oxide cathode

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The oxide cathode is a type of hot cathode in electron tubes which, thanks to its special design, has a high emissivity even at comparatively low temperatures. The oxide cathode is common in tubes for radios , amplifiers , tape recorders , hearing aids and televisions . Only tubes for very high power or special purposes such as noise diodes use a tungsten cathode.

Macro shot of a directly heated DAF96, the white layer is the finished oxide cathode.

The oxide cathode is operated at approx. 1000 K and therefore glows in a dark red glow. Some (mostly directly heated) oxide cathodes are so filigree that their glow can only be made visible in the dark.

history

The first oxide cathodes were the so-called barium vapor cathodes. The anodes of these tubes have a chamber with a certain amount of barium azide or an aluminum-barium compound at a suitable location. This chamber was heated by eddy currents during the manufacturing process. The barium released as a result was deposited on the colder parts of the system, including the filament. The layer thickness of 0.1… 5 µm was very thin, however, so that the service life of these cathodes was limited.

The Telefunken RE48 from 1923 was one of the first triodes with an oxide cathode.

From 1926, La Radiotechnique introduced the barium nitride process. The pure tungsten filament was coated with copper and oxidized at 800 ° C. Barium nitride was painted on the anode. When the tube was pumped out, the anode was heated to 500 ° C. The nitrogen escaped and barium remained on the anode. Finally, the anode was heated in the induction furnace and the cathode was electrically brought to 1200 ° C. The barium from the anode evaporated and reacted with the copper oxide from the cathode to form barium oxide and barium.
This process was later abandoned and the direct coating of the cathode, which is still common today, was chosen: This means that no barium remains on the cooler system parts. B. reduces the grating emission .

From 1927 z. B. in types RE134 and RE034, the cathode is already manufactured using the modern coating process.

Construction and manufacture

The components of the cathode layer are barium oxide , often also strontium oxide in a mixing ratio of 1: 1 or 1: 3.

During production, the cathode layer is applied as a suspension of a binding agent with the carbonates of the starting materials using suitable measures ( spraying , dipping , electrophoresis ) in a layer thickness of 20 ... 80 µm on the carrier material. The carrier material is either the heating wire made of nickel or tungsten (so-called direct heating) or a nickel and more rarely a copper tube in which the heating wire heats this tube in an electrically insulated manner (indirect heating). For heating, see electron tube: heating .

In the further course of the tube production, the cathode is formed ; this happens when the tube piston is closed on the vacuum pump. When heated in a vacuum, the carbonates break down into oxides ; hence the name oxide cathode . In the second step, baking , an electron current is generated by further heating together with the application of appropriate voltages. This baking initiates an electrolytic process that ensures a small amount of pure barium in the layer.

The finished oxide cathode layer does not emit uniformly everywhere. There are numerous small zones in which there is very good emission, while outside these zones there is less good emission.

Telefunken made experiments with foil cathodes in the early 1960s. The oxide suspension is applied to a plastic film with low thickness tolerances. After drying, the oxide layer is mechanically detached from the carrier material, punched out using suitable stamping tools and at the same time pressed onto the (angular) cathode tubes under high pressure. The advantages of this process are very small tolerances in the layer thickness down to 1.5 µm, a lower surface roughness and a sharply delimited emission range. The former is a prerequisite for tension grid tubes with their extremely small distances between the cathode and control grid .

business

When operating at normal heating temperature, a small part of metallic barium constantly evaporates, and electrolytic decomposition also continues. Too much or too little heating disturbs the equilibrium of these processes. Therefore the heating voltage of the oxide cathode should be kept constant at ± 5%.

Underheating promotes a depletion of emission centers by slowing down the electrolytic process, while overheating promotes evaporation of metallic barium from the cathode layer.

literature

  • Heinrich Barkhausen: Textbook of electron tubes, Volume 1: General principles . 11th edition. S. Hirzel Verlag, Leipzig 1965.
  • Herbert G. Mende: Radio tubes, how they became, what they do, and other things that are not in Barkhausen . Franzis-Verlag, Munich 1966.
  • Gerald FJ Tyne: Saga of the Vacuum Tube . 1st edition. Prompt Publications (Sams), Indianapolis 1977, ISBN 0-672-21470-9 .
  • Fritz Stork, Waltraud Wegner: The foil cathode . In: Lothar Brück (Ed.): The Telefunken tube . No. 43 . Franzis-Verlag, Munich 1963.
  • Ludwig Ratheiser: The great tube manual . Franzis-Verlag, Munich 1995, ISBN 3-7723-5064-X .

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