I m trying to understand the cons of using a LaB6 cathode over dispenser cathode. Since dispenser cathodes are low temperature cathodes where as LaB6 is high temperature. Is this correct to state that higher the cathode temperature , more is the velocity spread of electron? Can anybody suggest?
Dear Anjali, are you asking your queston in regard of TEM application?
and what dou you mean by dispenser cathode?
Dear Elif Tarhan Bor mine application is microwave tubes. So the output power from this tube would be extremely high about 1MW. In that context, the choice of cathode material matters. Dispenser cathodes are oxide cathodes which are maintained in a kind of reservoir.
I guess you can find your answer by understanding the principle of thermionic electron sources.
Electrons can be given sufficient energy to overcome the natural barrier, which prevents them from leaking out from the surface, upon heating any material to a high-enough temperature. By the way, this barrier energy is known as Work function (few eV).
Physics of thermionic emission is expressed by Richardson’s law (if I’m not wrong by the name) by relating the current density from the source to the operating temperature. (You can find this relation in any TEM books.) In other words, You either need high melting point materials, i.e. W that had been used in TEM’s for long years, or low work function materials, i.e. LaB6.
Although heating up a thermoionic source provides high current density, higher temperatures shorten the the life of the source. You should also consider increased price and higher vacuum requirements LaB6 vs W. Further increased current densities can be obtained by field emission. This is even very expensive for TEM’s so I can not imagine in use of microwave tubes.
Increased current densities brings the question of stability of source as well.I hope my answer give you an idea. Kind regards. hope my answer give you an idea.
Thank you @Elif. I would like to add that LaB6 has high operating temperatures and also large life time.
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