Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Thu Jul 16 2015, 10:10AM

Looks interesting. I can't find a supplier this side of the pond, though.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Conundrum, Sun Jul 19 2015, 04:01PM

There was something about using carbon nanotubes as an emitter, the idea was that each vertical tube was a separate emitter in its own right.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Sun Jul 19 2015, 04:27PM

Hello Ash,

Having finished a few other projects, I'm starting to again think about building some kind of modest electron accelerator, and one of the issues is cathode design. I was considering using carbon nanotubes from this place:

Then I saw the "reticulated vitreous carbon". But how to use it? I'm wondering if it can be copper electroplated and then soldered, for example. I suppose I'll just have to order some of the material from McMaster and experiment with it.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Sun Jul 19 2015, 06:09PM

The first idea that came to my mind was some form of high current HV triode, using a LARGE anode. Some form of high current HV diode should also be feasible.

I'm assuming 'active pumping' during use, for simplicity and ease of modification.

I imagine a fairly high filament current and voltage may be required.

If you order some, JP, could you order some extra, and post me some? I'll cover the cost.

EDIT: Apparently it's quite easy to nickel plate carbon, so if you plate the ends with nickel, then copper plate the nickel, it should be solderable.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Sun Jul 19 2015, 08:00PM

One of the papers I saw that got me interested in carbon/graphite cathodes was this one:


In sec. 2.2 the authors describe what they call a "pyrolytic fiber carbon" which is used as a field emission cathode in their accelerator. I'm not sure exactly what the cathode material is, but it seems to have exactly the characteristics I'm looking for.

When I was searching and trying to find out what they were using, I came across the RVC material, and as a cathode it seems to have functional properties similar to cathode described in the paper.

Also I recently sent an email to Xin Nanomaterials asking some questions about the use of their carbon nanotubes in FE cathodes, so depending on what I learn about their CNT products, I will either get some nanotubes from them or I'll get some of the carbon foam from McMaster, or possibly some of each. When I get some of the material I'll let you know and I'll gladly send you some (gratis).

(BTW as far as the ebeam "diode" part of it is concerned I'm leaning toward something like this:

or like this:
, where the diode operates in a pulsed mode with a short cathode-to-anode gap, and the volume to be evacuated is relatively small).

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Mon Jul 20 2015, 12:31PM

By 'cold cathode', do I take it that there is no filament current required?

The pulse generators I've been looking at recently have used 'inductive voltage adder' technology, with fast SiC MOSFET's to produce pulses of under 100nS, similar to this one:

And this one although the SANDIA one is probably a bit big for anything we have in mind.

Another project I'm working on, now and again, involves placing a CW multiplier inside a vacuum chamber to contain/control the high voltages (several hundred kV), but that's a pretty conventional LINAC.

Max power allowed this side of the pond before a license is required for an accelerator is >1MeV, but I believe it's higher your side of the pond.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Sat Jul 25 2015, 03:25AM


Yep, it's pure field emission. Depending on details, emission starts at some threshold value of the electric field (magnitude) at the surface, and the emitted current rises exponentially as the field strength increases. As the field strength continues to increase, the current and emission current density continue to increase and at some point a part of the emitting surface starts to vaporize, producing a plasma, which is a prolific electron source itself. This mode of operation is called "explosive emission".


I've been trying to come up with practical ideas to get up into the multi-megavolt range. When I think about it, most of my ideas seem to be some variation of an "inductive voltage adder". One of these days I'll start a thread to discuss some of the ideas.


Speaking of that, I'm trying to find out if Siemens ever built one of their proposed novel electrostatic accelerators as shown here:

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Sat Jul 25 2015, 09:36AM

I've seen the Siemens accelerator before, not sure if it's actually been built, though.

Having followed Klugesmith's experiments with his electro-magnets, I've started toying with the idea of 'conventional amateur cyclotrons', but I'm not convinced that the cyclotron format is suitable for some of my ideas.

There is a lot of interesting stuff over on the 'Cyclotrons' forum, though.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Fri Jul 31 2015, 01:10AM

I wonder, how high in voltage is it possible (maybe "practical" is a better word) to go by brute force; e.g., using a dual resonant pulse transformer (DRPT) directly driving an electron beam diode?

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
johnf, Fri Jul 31 2015, 09:12AM

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

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Fri Jul 31 2015, 06:50PM

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.?

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Fri Jul 31 2015, 10:46PM


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

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
johnf, Sat Aug 01 2015, 04:15AM

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

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Sun Aug 02 2015, 08:06PM

How did you apply the CNTs to the surface of the cathode?

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Tue Aug 04 2015, 11:48PM

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.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Wed Aug 05 2015, 09:28AM

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]"

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Thu Aug 06 2015, 02:23AM

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.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Thu Aug 06 2015, 10:58AM

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.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Thu Aug 06 2015, 09:28PM


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


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.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Conundrum, Sat Aug 08 2015, 07:29PM

I am working on an intriguing modification of my own design to the VFD tube that uses a similar effect.
In this case the extra efficiency comes from using a mildly radioactive source (namely 241Am) under pyrolytic graphite in low pressure helium (due to the electron mean free path)
I worked out that by doing this the hot filament could be avoided entirely as the burst of electrons caused by those alphas emerging from the PG surface would allow the tube to run in "cold cathode" mode down to <10V at <1mA comparable in efficiency to OLED.

-A

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Fri Aug 14 2015, 01:14AM

Here's an interesting paper regarding a study of the properties of several types of cathode materials (explosive emission mode):

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
jpsmith123, Tue Sept 01 2015, 07:05PM

I ordered some RVC material from this place:

Once I get it and start working with it I'll post an update here.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Ash Small, Thu Sept 03 2015, 10:50AM

Interesting paper, I'll have to re-read it, but it looks like plain old CF could be a very useful cathode material as well. It's certainly something I'd like to investigate.

Re: Reticulated Vitreous Carbon as a Field Emission Cathode
Conundrum, Sun Sept 06 2015, 06:43AM

Same here.

I did read somewhere that pyrolytic graphite (ie solid not plastic type) if chemically etched first to expose the surface graphene makes quite a good emitter.