a) Can you draw a sketch of an activated emitter wire assembly?
b) What is the purpose of activating emitter wires? How does it change the emitter’s surface?
The electric field strength on the emitter can greatly be enhanced by creation of dendritic microneedles (whiskers) on its surface. This process is known as emitter activation. (Chap. 8.3.2)
c) What is the general procedure to effect emitter activation?
The pyrolytic decomposition of carbon-rich organic molecules on the surface of a tungsten wire results in needle growth. The needles are highly branched and consist of tungsten and carbon in variable ratios.
d) How does activation affect mechanical strength of the emitter?
Activated emitters are better described as ceramics than as a metal wire. They have low mechanical strength and require careful handling. Nonetheless, they can be heated up to 1200 °C and withstand most chemical attacks with ease.