Schottky emission cathodes

Schottky emission cathodes consist of zirconium-doped tungsten tips, with a radius of about 0.5 -1 pm, coated with a ZrO layer. This layer decreases the work function from 4.5 to 2.7 eV. A Schottky emission cathode works with a higher electric field strength at the tip that decreases the work function by A

Schottky effect. Fig. 78. curve b) and concentrates the emission at the tip with a virtual electron source having a diameter of 15-20 nm. However, the electrons still have to... 

The surface of the sp -carbon film was examined as a cold emitter of electrons in a normal applied electric field. Figure 11.18 shows the volt-current characteristic of a vacuum diode with a flat sp -carbon cathode at room temperature. The figure is plotted in the Schottky coordinates log(7) — where U is the applied voltage. The distance between the anode and cathode was about 0.3 mm. As can be seen in Figure 11.18, the electron emission from the cathode can be described by the Schottky law ... 

The current density jj s for this process, which at high temperatures is also called glow emission, was originally calculated by Richardson for glow cathodes in vacuum valves and, taking the Schottky effect into account, is given by ... 

Figure 78. Potential at the cathode-vacuum interface for W and LaB(, thermionic emission, ZrO-W Schottky emittsion. and field emission guns...

Two classifications of field emitters are used in electron microscopes, cold and Schottky sources (Table 7.2). Whereas the former operates through Fowler-Nord-heim electron tunneling " from a cathode wire held at room temperature, the latter features thermionic emission from a Zr02-coated sharpened W filament at 1,800 K. In both cases, an electrical field draws electrons from the narrow filament tip into an... 

We now turn attention to conditions at the electrodes. These play vital roles in establishing the pre-breakdown conditions in the liquid under high electric stress and in triggering the breakdown itself. It has been natural to invoke electron injection at the cathode as an important component since high fields will lower the potential barrier to electron transfer across the interface whether it occurs by a thermally activated or tunnelling process. However, employment of the Schottky formula for field-assisted thermionic emission or the Fowler-Nordheim one for tunnel emission which are appropriately applicable only for electron transfer to a vacuum is a much too simplified solution to the problem. 

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