Quote:
Originally Posted by Alastair E
The article was very specific....
60/40 Thorium/Barium Oxide Coated Cathodes--as used in the majority of CRT's--and certain Electron-Microscopes.....
The art was to keep them hot therefore attack from moisture eliminated.
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Yeah, most EMs don't use thoriated or chemically treated/oxide filaments - its all pure bare metal. Perhaps some do but the that's not the norm. The mission of an electron microscope typically is to produce a very very fine spot of electrons in the range of nanometers or less at beam currents of up to the range of 100 nanoamps or so. Very different from CRT that need gobs of current to bombard large phosphor screens, trade off is a much larger spot size. So, EMs have very different service compared to the CRT or kinescope used in television.
Filament life in an EM is typically 50 hours or so and the vacuum equipment is designed for easy filament replacement using simple tools. The short life is also a function of cycling EM filaments by exposure to air at lower vaccum (poor vacuum) 10-4 torr to 10-7 torr conditions without the use of getters or baking. LN2 (liquid nitrogen) traps are somtimes used to help remove water vapor. Unlike the EM, a CRT design typically requires complex sophisticated equipment to replace the cathode emitter and gun assemblies need to flashed with getters. Assembly also requires baking to outgas everything inside the envelope - mainly to drive off water vapor and dissolved gases quickly. However, the payoff, if done right, is an extremely high vacuum 10-8 torr or better, with continuous getter activity over the service life of the CRT, which in some cases can be decades - if you get all your sealing surfaces air tight! That's a big 'if' with frit and glass to metal seals.
Most SEM and TEM (scanning and transmission electron microscopes) use solid tungsten wire filaments. The wire is bent sharply to form a tip to form the cathode emitter. For smaller spot sizes and higher brightness (more electron emission) EMs can use LaB6 (Lathanum Hexaboride) emitters. The LaB6 has a unique ability to physically have a sharp tip compared to a tungsten filament that when heated has a fairly low work function for very fine stream of electrons. In this configuration, a single crystal of LaB6 is supported and heated by the tungsten wire. Heat from the tungsten wire causes the LaB6 to emit thermionic electrons.
As an fyi. Pure tungsten and LaB6 cathode filament heaters are often held by posts of MO and RE for support. The tungsten wire is spot welded to the substrate posts, which also acts as conductors for the filament current used to drive up the temperature of the tunsgten. As used in electron microscopes they are typically stored at atmosphere and long term storage. They are not affected by temperature or humidity. It's not required to keep them in a desiccator. Tungsten and LaB 6 will rapidly outgas when gradually heated initially prior to bringing the emitter up to full operating termperature. That process may take several hours.
Here's a link to various emitters used in electron microscopes (SEM and TEM)
https://www.google.com/search?q=lab6...Q&ved=0CCoQsAQ