P2 iron
P2-iron or P2-sheet is a composite material made of iron -, and aluminum sheets , by plating are joined together mechanically and by subsequent treatment by higher temperatures, a chemical compound received.
PN iron or PN sheet is plated on one side with nickel instead of aluminum.
For low-power electron tubes, P2 iron is the standard material for anode and shielding plates as well as cooling blades on grids .
history
In the years before the Second World War , consumer goods such as cigarette boxes and food containers were made of aluminum- plated iron (trade names Ferran or Triwalith ). Aluminum corrodes less than iron, but is comparatively expensive to manufacture, which is why it seemed desirable to save Al without sacrificing the stability and corrosion resistance of the objects of use.
When this composite material is heated to red heat, the Al-plated side becomes rough and dark in color while it bonds with the iron base material. Further tests have shown that this material is extremely useful for vacuum technology, especially at higher temperatures such as those found in electron tubes during use.
With the outbreak of the Second World War, nickel, which had previously been used as a material (among other things) for anodes in electron tubes, became scarce; it could only be obtained through imports. The carbonization customary for tubes with a higher output to increase the heat radiation had disadvantages in terms of vacuum technology. The P2 iron therefore became the most widely used material for anode sheets for low-power tubes on the European continent over the next few years. After the war it was also used in the USA.
Manufacture and use
P2 iron is an excellent material for anode sheets because
- The aluminum layer prevents the constant outgassing of oxygen from the pure Fe sheet in a vacuum and at high temperatures (vacuum deterioration over the service life),
- The formed on aluminum in air, aluminum hydroxide layer impedes the evaporation of aluminum at operating temperatures and thus prevents adverse effects on the chemistry of the cathode electron tube,
- the rough, comparatively dark surface has degrees of blackness of up to 80% of the total radiation capacity of the black body and can therefore more effectively radiate thermal energy introduced by the anode power loss .
In addition, the raw materials are still cheaper to procure than the nickel that was used exclusively before the war. Another advantage is that the harmful gas content of a P2 sheet metal anode is only a fraction of that of a blackened nickel anode of the same size (because of better heat radiation).
Disadvantages compared to nickel are the higher surface sensitivity to chemical effects (gloves and high workplace cleanliness during assembly!) As well as increased storage costs, since the cut edges in particular expose the iron in the core directly to the oxygen in the air and the mostly existing humidity causes rust favored.
The total thickness of the sheets is usually 0.15 mm, the aluminum layer (before the reaction) on average approx. 10 µm. Thicker layers carry the risk that pure Al remains during the reaction and locks in the resulting crystals, which impairs the radiation properties of the layer.
From approx. 430 ° C, the very pure components aluminum and iron form a microcrystalline, highly porous structure made up of tiny FeAl 3 particles. The reaction is strongly exothermic. In order to obtain optimal surfaces, the blackening should take place under protective gas or vacuum and the Al should have a clean surface. The resulting blackened iron is slightly more brittle than when it was bare. With an optimal reaction, no alloy is formed at the interface that would impair the heat conduction properties.
Due to (among other things) a certain oxygen content and no silicon impurities in the Fe as well as small additions of silicon with the Al being completely free of zinc , the initial reaction temperature can be raised to 680 ° C so that the blackening does not yet form during the stress- relieving annealing of the mechanically finished parts. Heating above 900 ° C should be avoided, as otherwise the crystals will sinter together , thereby reducing the roughness of the surface and thus worsening the radiation properties.
According to Telefunken , the fully formed anodes are cleaned in trichloroethene and installed in the tube systems in a bare state without further thermal outgassing. The blackening only takes place in a later manufacturing step on the vacuum pump with the simultaneous thermal outgassing of the entire tube system by heating in the high-frequency field to the necessary conversion temperature of approx. 680 ° C. As a desirable side effect, the rough surface acts as a getter when it is formed .
PN iron
In tubes with very small distance between anode and cathode (. Eg tube diodes ) on one side only Al-plated iron sheets are used: From the layer ausdampfendes aluminum would be with the oxide cathode connect and undesirably the work function increase. The inside of the anode is plated with nickel, there is no blackening. This material is known as PN iron or PN sheet .
literature
- Werner Espe: Materials science of high vacuum technology . 1: Metals and metallically conductive materials. VEB Deutscher Verlag der Wissenschaften, Berlin 1957.