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Reticulated Vitreous Carbon as a Field Emission Cathode

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johnf

Fri Jul 31 2015, 09:12AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

I have played with carbon based field emission diodes some time back.
They tend to allow carbon to migrate to the anode making them bi-directional. Oxford offered x-ray tubes based on this technology but they have been withdrawn I wonder if they had the same problem.
I got down to 0.2 volts per micron separation anode cathode to enable field emission. I was getting around 10mA @ 20kV for hours on end very stable after initial burn in

jpsmith123

Fri Jul 31 2015, 06:50PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Hello Johnf,

That's very interesting. I'm wondering, what specific type of carbon based material were you using as an emitter, e.g., nanotube film, pyrolitic graphite disk, etc.?

johnf wrote ...

I have played with carbon based field emission diodes some time back.
They tend to allow carbon to migrate to the anode making them bi-directional. Oxford offered x-ray tubes based on this technology but they have been withdrawn I wonder if they had the same problem.
I got down to 0.2 volts per micron separation anode cathode to enable field emission. I was getting around 10mA @ 20kV for hours on end very stable after initial burn in

Ash Small

Fri Jul 31 2015, 10:46PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I was wondering about this after reading about 'explosive emission', or rather, I was wondering what happened to the 'sputtered' carbon.

This wouldn't affect it's use in 'triode mode' though, would it?

Incidentally, the last job I had before I stopped work was for Oxford (OIPT). A significant amount of my vacuum equipment. etc. originated from their skip/duimpster, with the appropriate 'scrap ticket', of course

johnf

Sat Aug 01 2015, 04:15AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

jps
I was making multiwall and single wall CNT's by arc discharge @ partial pressure then purifying them by oxidizing any amorphous clag at 500 degrees C in air.
ended up with 90+% pure CNT

jpsmith123

Sun Aug 02 2015, 08:06PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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How did you apply the CNTs to the surface of the cathode?

jpsmith123

Tue Aug 04 2015, 11:48PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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After doing some more reading, it seems that the composition, manufacturing method and processing of CNT paste (after application to the cathode substrate) is critical to the performance and reliability of the resulting emitting surface (and the substrate material may be an important variable with regard to adhesion).

I'm thinking that commercial FECNT pastes (available from Xin Nanomaterials and possibly other manufacturers) are most likely proprietary.

Also IIRC, somewhere I read that nanotubes are not good to get into your lungs, i.e., you don't want to inhale any kind of dust which may contain nanotubes.

In light of this, I think I'm just going to get some "carbon foam" from McMaster and see if it can be electroplated and soldered.

Ash Small

Wed Aug 05 2015, 09:28AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Aren't the CNT's produced by evaporation in a vacuum, or something?...Or am I getting confused with Buckminster Fullerines, or something?

Did JohnF mean he was using the 'explosive emission' process to produce the nanotubes, or am I way off the mark?

I seem to remember reading something about a 'sputtering' process used to produce these things?

EDIT: "Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps, that was intended to produce fullerenes.[78] However the first macroscopic production of carbon nanotubes was made in 1992 by two researchers at NEC's Fundamental Research Laboratory.[79] The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates because of the high-discharge temperatures.

The yield for this method is up to 30% by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects.[43]"

jpsmith123

Thu Aug 06 2015, 02:23AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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The phrase "explosive emission" refers to a mode of electron emission from a surface, where, due to a very high electric field, some of the surface material is vaporized (it doesn't have to be carbon/graphite) and the vaporized material becomes an ionized plasma, which is itself a prolific source of electrons.

Johnf impliedly answered my question by mentioning how he made nanotubes, i.e., by way of the so-called "arc method" (vis-a-vis other methods such as CVD, laser ablation, pyrolysis, etc.), but this has nothing to do with the end use of the nanotubes as an electron emitter in some particular case.

Ash Small

Thu Aug 06 2015, 10:58AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Maybe I mis-read it, JP. it wasn't clear to me that John was applying the CNT's to cathodes.

I do have some ceramic paste somewhere that's used in vacuum systems, I'm not sure how suitable it would be for mixing with CNT's, but it may be a good starting point. I'll try to find it later, I can't remember the name offhand.

jpsmith123

Thu Aug 06 2015, 09:28PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

wrote ...

Maybe I mis-read it, JP. it wasn't clear to me that John was applying the CNT's to cathodes.

I know. He didn't explicitly state that, but that's what I inferred.

wrote ...

I do have some ceramic paste somewhere that's used in vacuum systems, I'm not sure how suitable it would be for mixing with CNT's, but it may be a good starting point. I'll try to find it later, I can't remember the name offhand.

I had almost the same idea. I wondered if you could make a FE cathode by mixing nanotubes with alumina potting compound like Cotronics #801, or something like that.

It might work, but as I see it, with the different variables involved, making a robust and reliable FE cathode would seem to be a major development project in itself, and I'm looking for something as close to off-the-shelf as I can find.

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