Every neon sign electrode I have seen has had what appears to be a deep-drawn metal tube with one end domed shut and one end open, with some sort of white (ceramic?) bead ring thing crimped into the open end. I have the following questions:

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1) Material:
What are the electrodes made of? When you "bombard" them with hundreds of mA to get them yellow hot, what is actually happening? I assume they are made of some sort of getter-like material (like titanium) that absorbs impurities when very hot, but I might be wrong.

2) Current Ranges:
I have seen catalogs for some neon sign electrodes, and they appear to have a minimum current rating. Do they need to maintain a certain temperature to work right, or is that just a size suggestion from the manufacturer? I thought they were just cold cathode, but maybe there is something I am missing.

3) White Bead Ring Thing(?):
There always seems to be a white ring crimped inside the open end of the electrode. What is it? I wouldn't be surprised if it was some horrible radioactive ceramic material to aid with striking, but I have no idea.

4) Discharge Shape:
Why does the discharge only occur inside the cavity of the electrode? Why not on the outside (visible part) of the electrode as well?

5) End-of-life:
Do they ever die? If they are indeed cold-cathode, I would assume the tubular shape of the electrode means that sputtering is kept to a minimum (sputtered material just jumps to the other side of the electrode), but as long as there is no special coating on them they should last practically forever if the walls of the metal tubing are a reasonable thickness (not tin foil).

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Thanks so much!
 

1. Nickel plated pure iron usually. Almost all electrodes made today are also activated - that is coated on the inside with barium carbonate and probably strontium and calcium carbonate, and there may also be variations as fluoride and peroxide. When you bombard the electrode, two things happens - metal electrode shell outgases in the vacuum and carbonates decompose to oxides. Oxides are next getting ready to do some poorly understood magic ) which probably means oxide reacts to electrode metal, reducing to a minor amount of free barium which migrates to the surface and makes actual cathode activation. Older Soviet tubes used simpler electrodes - just pure iron shell, slightly oxidized in the air. It is speculated that this oxide worked as poor man activation.

2. No minimum rating. You can run cold cathode at as low current as you wish with no penalty except low brightness. Running above ofcourse reduces tube life.

3. Pretty benign ceramic. The purpose is to deter the discharge from electrode shell edge and external surface, that way reducing sputtering and increase lifetime.
 
4. That is the great invention of french man Claude! This is a hollow cathode. The idea is that although cold cathode discharge inevitably sputters a lot of cathode material, with a hollow cathode the material sputtered just lands inside the hollow shell nearby on the cathode surface and forms a material cycle, and can not land on the glass nearby. This almost completely stops cathode material loss and prevent the metal to settle on the glass, which captures inert gas and limits tube life. I also guess that ion stream flowing into the shell opening pushes sputtered atoms back. Ceramic collar with a small opening probably is also helping with this. Metal to inner tube glass distance is chosen small enough so discharge avoids external electrode shell surface until electrodes are especially badly made or processed.

As you can imagine, if the discharge is allowed on the external shell surface, all the great advantages of hollow cathode are negated and sputtered metal mirror quickly forms leading to fast gas capture and short life.

5. That depends on filling gas type. Pure neon tubes have a finite life determined by gas capture. Poorly made tubes - short, improperly bombarded/activated and underfilled with neon (this is very critical!) suffer.  Mercury tubes in fact live a long long life, as argon capture happens rather slowly, and heavy mercury atoms greatly suppress sputtering. Mercury tubes are generally getting impractically dim because phosphor ages and also mercury tends to react with anything dirt may appear in the tube over time, typically forming black compounds, which cause UV loss on phosphor grains. Badly made mercury tubes are also suffering from fast material sputtering from the electrodes causing dim ends.
 

@RRK

Thanks so much, that was very very informative!

What exactly do you mean by "cathode activation"? What benefit does this serve for the tube?

Emissive coating or as a verb, a process to bring emissive coating to work.

Without activation, cathode drop for cold cathode tube is in a range of ~200V. Activation brings it to about 100V and below.

 

@RRK
So this is an emissive coating that works cold? Interesting, I haven't heard of anything like that.

This is a subject of some speculation. Although it is known that electrode shell itself is relatively cold (said to be 200-300C), actual cathode spot is relatively small and definitely does not cover the whole electrode surface. Until activation works ok, that is, not poisoned for example. That leads to a suspicion that the surface of oxide coating may actually be relatively hot on a microscopic scale, and the emission, in fact may be at least partially thermoionic!

@RRK
Ah-ha! So it is sort of hot cathode-ish... Thanks so much, that answers all of my questions!

The term cold cathode is only relative compared to hot cathode lamps, and to our temperature scale.  A real cold cathode would maybe have to be at -273C and then thermal emission would stop!  But even at room temperature, the emitter coating of a typical FL lamp is emitting some 25 electrons per second.  Mildly raising temperature and the application of an electric field greatly increases the emission.  In a typical hot cathode fluorescent lamp with an emitter temperature of about 900-1100C, the emitter releases sufficient electrons to carry a discharge current of hundreds of milliamps, and for the 'cold' cathode variety their operating temperature enables a few tens of mA.

As far as I understand, there is actually a minimum current rating for cold cathode electrodes.  If they cool too much then the rate of emission available from the coating may decrease below what is required to sustain the discharge, and then electrons begin to be ejected from the metal rather than the coating, with consequent sputtering and life reduction.

In terms of operating temperature, we must remember that the emitter coating on the inner surface of the electrode has a kind of powdery texture, with only loose thermal contact to the metal substrate.  Due to its low thermal mass it is therefore possible that the coating has a rather higher temperature than would be measured for the metal shell itself.

Regarding the ceramic ring, it has a second important purpose.  During bombarding, the electrodes are heated to a dull red temperature.  If they are too close to the glass tube it can be cracked by thermal shock.  In Claude's original tubes from 1914 onwards, he applied rings of glass beads strung on metal wires to keep his then pure copper cylindrical electrodes sufficiently spaced from the glass.  Soon after that I think it was GEC-Claudegen in England, an associate company of Claude, that introduced the ceramic bush to combine that feature with those mentioned by RRK.  But it's difficult to find early literature on this, it could also have been one of the continental companies that worked out the present design.  It must be in the patent literature somewhere, but not so many people seem knowledgeable in the very early neon / CCFL tube developments.