What is a neon lamp's failure mechanism.

[schmitt trigger]

Take a common neon lamp, NE-2 for instance.

After a few years of operation, they start flickering, every time more slowly, until eventually they fail to turn on at all.

I have also noticed the the inside of the glass darkens.

What is going on here? Electrode sputtering?
Neons are glow discharge, and they don't require electrode coating on the electrodes, which could evaporate?

[T3sl4co1l]

Helium diffusion?

I never found out either, am curious.

I wouldn't think surface chemistry (adsorption/sputtering changing the surface energy?) would be able to hold off quite that much voltage.  I guess I'm unsure about emission sources -- it certainly seems likely (a source of ionization -- like ambient light -- helps an aging lamp), but I wouldn't expect any gross changes in work function or dark ionization from just the materials that are inside the bulb to begin with.

The ones that are seeded with radioactive material of course (not the usual lamps!) have a clear wearout mechanism.  AFAIK, Ni63 is the most common choice, which shouldn't drop significantly over the current life of most devices so far.

Tim

[Neomys Sapiens]

Unfortunately, the RAC Failure Mode Distributions do not show any separate data for glow lamps. The are semingly thrown together with the incandescent annunciator lamps, which makes the data not applicable. (Open, short, and anything related to filaments...)

All sources give higher lifetime for 220V indicator neons than for 110V types. Behind this is the bigger value and factor of influence of the limiting resistor on the operating current, which is very small when 110V and coated cathodes are involved.

O.P.Herrnkind in 'Glimmroehren und Kaltkathoden-Relaisroehren' states that electrode sputtering 'Elektrodenzerstaeubung' is a certain factor, which is proportional to the operating current. He also states that it is cathode sputtering which is causing the dark deposit. Further he states that the end of the lifetime of any glow-discharge devices is characterized by steadily raising ignition and maintaining voltages, with erratic changes also occuring. It is also noted that a lower-than-intended cathode current should be avoided too when the electrodes are oxide coated. The reason is that if the current is too low to keep the complete electrode area involved in the discharge, the non-emitting spots will passivate over time and not enter emission even if the current is increased again.

Mulder in his book 'Gas Discharge Tubes' (1963) states, that :
1.) cathode sputtering is dependent on current and gas presure, with higher pressure reducing the sputtering.
2.) cathode sputtering is the prevalent, but not the only effect which is determinating the lifetime. He names the 'cleanout'-effect, which he describes as being both physical and (to a lesser degree) chemical in nature, which causes the density of the gas filling to diminish. The first phase of this is happening, when ions encounter surface defects on the anode and/or the envelope. the second phase is characterized by sputtered deposits trapping additional gas molecules when forming the abovementioned deposit.
3) that SOME sputtering (during production?) is beneficient due to a getter effect, which binds non-noble gas molecules.
4.) that the envelopes, which feed a metal wire through glass without special considerations (Kovar etc. as well as design) are not to be regarded as reliably hermetically sealed.

This last point is exactly what I would have suspected to be the secondary failure mechanism, i.e. leakage and impurities, as thermal expansion and contraction will open ways for molecules to get out of or in to the tube's envelope.

[IanB]

It seems to be widely believed that operating neon lamps with a lower current extends their life, and a higher current reduces it. This may give some clue to the failure mechanism?

Empirically then, reducing the current would be a good idea.

[HighVoltage]

I was always under the impression that electrode sputtering cause the darkening of the glass, until we could not see the glowing anymore, although they are still glowing. My observation is, that they can last decades before they "fail".

[Gyro]

'Poisoning' of the cathode emmisive surface by sputtering from adjacent cathodes is also an issue, particularly in DC operated tubes like Nixies and Panaplex but it's hard to see how this could become a factor with the electrodes in an AC lamp (unless they sputter each other). My guess is change in gas pressure or binding of the gas in the electrode material.

I don't know if they include a small amount of Helium in Neon lamps to improve firing - if so, that's a very difficult gas to hold onto long term.

[Fungus]

I always thought it was a combination of factors, including the gas escaping over time (especially with cheaply manufactured ones).

I've always heard that current is a big factor though. Moral: Don't overdrive them.

[stranger]

I stumbled across a now forgotten art about 30yr ago in what was then a then old radar that used neon tubes (~1cm diam 2cm long) internally as voltage references, used rather like zenner diodes (current sources). This radar had "op amps" which were 4 tubes but worked just like any op amp today. These were inside sealed opaque boxes, and occasionally a neon had to be changed since it was no longer striking/regulating at 70v or 90V as per design spec. The old ones had to be stored for disposal, the storage box was marked "Radioactive". Eventually the embarrassing box was full and H&S contacted for disposal. After a lot of paperwork, thought and questions such as just how old were the tubes, we were told they were OK, just don’t eat them all on one sitting  The tubes were originally "radio active" with a tiny loading of tritium since they were intended to be used inside dark housings. Neon tubes normally need more than applied voltage to cause them to strike/avalanche and often this can be ionisation from a photon in an external source if light AND enough voltage are present, the tritium ionisation guaranteed startup in the dark. The "good" news was tritium has a 12.3yr half life and some of these tubes were already 2 or 3 times this age which was why they had become reluctant starters and why they could be safely disposed of. You may have seen a similar things about fluorescent tubes: http://www.irpa.net/irpa10/cdrom/00740.pdf