Material Source of Free Electrons

I am looking for a material that I can build into an experiment that, when stimulated, will emit electrons into a series-connected circuit.

Apart from radioisotops are there any viable options?

I was considering something like an off-the-shelf piezo element.

Does anyone have any suggestions?

Is there a list anywhere of materials that have loosely bound electrons?

Klaus Jensen

Reply to
Klaus Jensen
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On a sunny day (Sat, 09 Jun 2012 20:39:23 +1000) it happened Klaus Jensen wrote in :

Sounds like a radio tube (triode), or electron gun (CRT). Or break the glass of old CRT, use the electron gun or the indirectly heated cathode. You need a very high vacuum.

What are you trying to do?

Electrons are everywhere, rub some plastic foil...

Reply to
Jan Panteltje

A hot wire?

The material property you're looking for is "work function"; that's the ease with which a material will let go of electrons. Raising the temperature makes it easier for a material to let go of electrons (and, hence, corrode).

As mentioned, tube cathodes are made of materials with low work functions. But -- they don't work well in air, because the free-travel path of an electron in air is astonishingly short. That's why vacuum tubes are _vacuum_ tubes.

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Tim Wescott

Search the electron microscope area.

When we were doing e-beam sources, there were 'cold' sources - pretty unreliable generators and, 'hot' sources - much more reliable generators.

It is my understanding that these emitters MUST be in a vacuum, else the electrons get 'returned' to the surface, and then they almost look like 'non-emitters'.

I know many of the materials may be listed somewhere wtih NASA you can start

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a sample location there for finding outgassing is

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don't know where emitters would be, but I 'think' that's where the materials came from

From memory, hot tungsten was a good one, but there were quite a few 'cold' ones.

You might find it at the SUPPLIERS to these SEM people, too.

Reply to
Robert Macy

Hmm. Do you have access to the other end of this circuit? If so, I'd suggest inserting a battery. Otherwise the electrons are going to pile up in your circuit, resulting in a high negative potential which will stop the flow of more electrons in. Unless you push very hard.

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Paul Hovnanian P.E.

Would the emitter from a "cold cathode" fluoro tube have any special value for my applicatiion?

As is, it's in a vacuum, and requires higher voltage, but the heating requirement is eliminated.

Klaus Jensen

Reply to
Klaus Jensen

Yeah I was going to suggest a battery or photodiode as source of electrons. I've used a piezo 'sparker' the Zap stuff and check the ESD protection.

George H.

Reply to
George Herold

On a sunny day (Sat, 9 Jun 2012 19:05:58 -0700 (PDT)) it happened George Herold wrote in :

Piezo sparkers are fun, there even exist piezo transformers.

Reply to
Jan Panteltje

Piezo transformers? I've no idea how much current/ voltage you get from a sparker. George H.

Reply to
George Herold

It's not so much the electrons not getting away as the positive ions they generate bombarding the emitting surface

Tungsten filaments are fine, but need to be replace pretty frequently

- the tungsten atoms evaporate.

Heated lanthanum boride crystals last a lot longer - in a good vacuum

- and are much more stable.

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For a really bright source, you can't beat a cold field emission tip, but the single atom at the point of the emitter is very vulnerable to positive ion bombardment, so they aren't stable and don't last long enough to be practical.

Heated field emission tips aren't quite a bright, but they are more stable and do last longer. Carbon Nanotubes (CNTs) have got into the picture.

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I don't know if you can yet buy anything made with carbon nanotubes.

You might ...

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

On a sunny day (Sun, 10 Jun 2012 06:16:21 -0700 (PDT)) it happened George Herold wrote in :

Piezo transformers in priciple work like this pivot point ==============0====================================== metal arm | | --->[ ] piezo transducer, [ ]---> Uout Uin | electrickity to length change | piezo transducer, length change to electrickity =========================================================== metal base

In reality often a bit different:

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Reply to
Jan Panteltje

One possibly useful fact is that free electrons are stable in pure nitrogen, i.e. they don't form negative air ions. So a corona point running in a pure N2 atmosphere can give rise to some interesting physics.

Cheers

Phil Hobbs

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Phil Hobbs

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Interesting. Any papers, or URL's for papers? send directly if too OT.

Reply to
Robert Macy

yes! hexaboride. stability not from source, but damage back to source.which is exacerbated in 'poor' vacuums. Even at 10-9 torr

Reply to
Robert Macy

Yes, should work. I know I'm 'preaching to the choir' but remove ALL oil/dirt anddon't TOUCH anything going into that vacuum.

Reply to
Robert Macy

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I used to use a relatively gentle piezo sparker called a Stat Gun. Sold to 'blow' dust off vinyl records, it floods charge around 6kV gently across the platter, I use it to test for ESD sensitivity WITHOUT killing the product, well too much. When I couldn't find that I went to a push button starter for a gas heater, button 12 inch wire electrode, that was around 18kV and was extremely robust, could draw an inch arc, and killed any sensitive electronics, but at least showed you there was a problem. Oh, yeah, watch out, because you're part of the circuit ...from experience. I recommend Keytek, instead, more control. A bit off topic, but information: be careful electronics can be sensitive in a 'range' of voltage breakdown, not just 'above' some level. 4-6kV can disrupt, yet 10-12kV does nothing. Catches a lot of newbies off guard.

Reply to
Robert Macy

Odd place to reference, but:

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question #25 says N2 has -2.2eV affinity and 15.58eV ionization.

In pure N2 at STP and assuming Maxwell-Boltzmann statistics (typical for a low density electron gas), electrons will be free to travel, much like electrons in a semiconductor, at energies below 2.2eV. At these low energies, elastic collisions scatter and thermalize electrons, so that the average energy is around 26meV, and diffusion dominates. (Of course, it's remarkably difficult to dope a gas, so good luck making a semiconductor junction! A difference in work function on electrodes should provide the usual results, however.)

The momentum transfer of electron to nitrogen molecule will induce a gas flow in the direction of electron flow, the applied electric field. That is, "ionic wind" should occur without any ions being made, just momentum transfer. The effect should be somewhat weak, because electrons don't transfer much momentum to a beefy nitrogen molecule -- the mass ratio is about 56,000:1, like being hit by dust clouds blowing in the wind. (The "clouds" will actually be a lot bigger than nitrogen, because electrons at ~26meV are ~4nm across, while N2 is on the order of N's atomic radius, ~0.1nm.)

To achieve energies greater than ~26meV, you need an electric field much more intense, to overcome thermal scattering. When this is achieved, electron mobility should go up, because they are shooting past the nitrogens more often. Resistance will still be significant, because the mean free path is still small.

At 2.2eV and above, electrons will begin to glom onto nitrogen molecules: this is normally an absorptive process, removing 2.2eV per event, but the charge doesn't go away, it just gets really heavy. With N2(-)'s floating around, they can bump into N2 and pass it around without anything really happening (I see no reason this wouldn't be a degenerate case), and presumably you get your 2.2eV back when the ion touches whatever electrodes you're using. If this occurs at high rate, it will again show up on mobility, this time by shrinking it dramatically due to the greater effective mass.

At energies approaching 9.8eV, N2 begins to dissociate. The free atomic N will ionize at comparable energies and add a mixture of ions to the mix (ions tend to be more stable than free radicals). Recombination will produce light (either multiple photons, or a single UV photon at 9.8eV). At energies approaching 15.6eV, N2 will also ionize due to collisions, and further breakdown will occur. In these regimes, the gas probably becomes luminescent -- i.e., a glow discharge or arc, while the conductivity rises exponentially (avalanche cascade).

Tim

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Reply to
Tim Williams

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Thank you for that description. SAVED

Does this effect/Nitrogen in anyway help explain the origins/stabilty of lightning balls? Or does that really require silicon[dioxide]?

Reply to
Robert Macy

This exactly only works in N2 (if it works at all), and it doesn't even necessarily work when the N2 is physically close to *anything* -- pure gas phase reactions have peculiar chemistry, with a lot of species that you wouldn't expect from experience with liquid or solid phase reactions.

All the ion and radical stuff is going to be strongly affected by impurities and surfaces, which interact at different energy levels. More energy levels means more ways to dissipate it, and once you start putting energy into different charged species, you get energy converted to motion, which dissipates as heat. The introduction of more species muddies the system and can lead to breakdown at lower energies.

A nice ~3mm arc, in air, only drops about 20V at >10A; the mean free path at STP being ~68 nm, the energy rise of any given charged particle between collisions is a teeny 20V * 68nm / 3mm = 0.45meV. (Most of an arc is still neutral atoms and molecules, and the actual MFP between charged particles is much higher. Still, you can see the energy per collision is small.) This looks like continuum (resistive) heating because the thermal energy is ~20kK = 1.7eV: each step between particles feeds in a little more energy into a thermalized system.

I don't know much about ball lightning. Its credibility is low, as you might know. For a persistent plasma ball to form, there has to be a significant power input and something to seed or confine it. Raw electromagnetic energy (induction -- ICP, microwave, you name it) is the best way to make this happen in the lab. There is no reason to believe a true plasma ball will persist without external power input for more than miliseconds. More likely the phenomena consist of something banal, like burning cardboard, caught in an updraft and burning in just such a way that looks suspicious.

Incidentally, if you want a *really* fast flash, use air. The spectrum sucks (heavy on the blue and UV side), but it's a lot faster than xenon because all the interactions allow energy to dissipate rapidly. It's pretty trivial to get sub-1us pulses with a high voltage, low ESR cap (like the kind Tesla Coilers like to make).

Tim

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Reply to
Tim Williams

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It's true that once the electrons hit some surface, they won't be free any more, but so what? A corona point will put out a few microamps forever. That's enough to do all sorts of interesting things with.

Cheers

Phil Hobbs

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Phil Hobbs

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