What are the little bits of metal that come out of light bulb filaments and why do they stick to only one terminal?

Question

Recently I changed the headlights in my car and took a few micrographs of the filaments. A couple composite photos of old bulbs:

For reference, here's a new bulb filament:

On the left wire in the images of the old bulbs, there are many metal flakes and hairs attached to the left wire. What are these little bits of metal(are they metal whiskers like tin whiskers), did they come from the filament or the wire, if from the filament how did they get to the wire, and why are they stuck there today?

I'm kicking myself for not measuring which end was connected to positive vs negative when I could get to everything.

Answer 1

Those are beautiful photomicrographs! Awesome to see those things up close!

What are these little bits of metal (are they metal whiskers like tin whiskers), did they come from the filament or the wire

They are whisker-shaped, but they didn't grow from the write. The metal atoms were sublimated from the hot part of the filament, likely as ions, and recondensed on the cold wire.

The electric field distribution around the wire has large gradients where sharp features are present. I imagine that this was causing the whiskers to grow in preference to other shapes: the electric field gradients are largest at the whisker tip.

if from the filament how did they get to the wire, and why are they stuck there today?

Once the filament is hot enough, the metal atoms have a reasonable chance of getting freed. They become an at least partly ionized cloud surrounding the filament, and will happily recondense on anything colder in the vicinity.

The electrical field distribution between the filament and the wire surely affects how and where they deposit.

I'm kicking myself for not measuring which end was connected to positive vs negative when I could get to everything.

You have a good hunch!

When the filament is hot, there also enough electrons freed from the filament that it acts like a hot cathode in a vacuum diode. If a high pulsed voltage is applied across the filament - e.g. if you were driving the filament with narrow, high amplitude PWM waveform - the light bulb acts like an avalanche vacuum diode. It breaks down, with visible arcing.

That's why we can't drive light bulbs with arbitrarily high amplitude PWM waveforms even if thermally the filament would have no problem with it. An operating light bulb is a bit like a tunnel diode connected across the filament resistance.

The filament is surrounded by a hot-cathode-emission electron cloud, with some metal neutral atoms and ions thrown in for good measure. There's vastly more electrons though.

Answer 2

This is a tungsten-halogen bulb, isn't it?

Normal (tungsten + inert gas or vacuum) bulbs don't have these crystals. Their glass gets uniformly black until enough tungsten migrates from the filament and then they fail.

Halogen bulbs, on the other hand, have the halogen cycle that returns some of the tungsten back at the filament.

This is good - the filament can be driven to higher temperature in order to be more efficient and still have longer lifespan. The process even self-regulates to some extent, by putting more tungsten at the hottest (i.e. the thinnest) part of the filament.

The halogen cycle carries some material to the metal parts inside the bulb as well. There, the crystal growth is slower and allows for building large, visible crystals.

Unlike the picture in the @Kuba answer, I don't think that the electric fields have some effect. First, they are not as much intense. The photo looks like 12V headlight bulb where the whole filament spiral is ~5mm long. The crystals at the wire on the left look more or less uniformly spread. The lower part of the filament is connected to this wire so one would expect much weaker field than at the other end. And the temperature is not high enough to really create ions. Evaporation goes as whole, neutral atoms.

p.s. The filament itself looks different from the new, its surface has distinct visible features. They are an artifact the same process, because the evaporation favors the small crystals and the deposition of the material back favors the large crystals. (This is almost always the case with crystals, no matter what the exact process facilitating the material transfer is.) As a result, the small crystals shrink and the bigger crystals grow until we have visible-size crystals on the wire surface.

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p.s.2 If you look carefully, dendrites don't grow only from the left support. Some minor amount could be seen at the right one as well. The simplest explanation is that their growth is temperature-dependent.

Dendrite crystal growth is a well-known phenomenon in electrochemistry and they are a major failure mode of many types of batteries. But they never grow off the positive electrode.

Answer 3

Others have explained the broad strokes of the mechanisms at play here, so I'll address the one noted peculiarity: growth on the connection rod / electrode.

This may simply be that, being longer, it gets hotter towards the end, and thus participates in the halogen cycle.

Leads are usually molybdenum, AFAIK, so are reasonably good (electrical and thermal) conductors, but a long one will have a higher temperature rise at the end. It probably has a higher temp rise along its length as well, due in part to radiation from the adjacent filament, and perhaps convection through the tenuous gas as well.

This seems at least partly supported by the amount of growth varying with position, being less at the top (cold end towards glass base); there also seems to be a line-of-sight element to it, hence the greater deposition adjacent the filament. (Or perhaps it's not actually much more material, but the deposits happen to be of a taller, flaky nature here?) The pressure inside these bulbs is not exactly a hard vacuum, but it may still be low enough to see a concentration gradient even over this short distance.

And, the other electrode does indeed have a very small amount of growth on it. So it's at least safe to say, it's not purely an electrolytic sort of reaction.

And, that's about the limit of my knowledge on this topic. I don't know offhand if thermionic emission or ionic conductivity plays any role in the halogen cycle. There are certainly atoms and molecules present which could be ionized (whether by electrical discharge, or thermal dissociation). Even if present, it does not appear to be the dominant material transport mechanism, at least.

Sodium for example is readily ionized even at mild flame temperatures -- hence the ubiquitous orange sodium glow when placing things into a fire (there's inevitably traces of sodium on things, for various reasons: sweat, dust, water, etc.).

But these are not sodium lamps, and I don't know what the ionization potentials or dissociation constants are for the species involved here. They may instead be rather stable molecules, which only break down autocatalytically on hot metal surfaces (and at rather high temperatures, at that!). If you have additional curiosity in this direction, perhaps the Chemistry Stack would have some thoughts on it.

Answer 4

Others have covered the physics of what's going on here really well, so I'll only add this as a good example.

A very related phenomena to what your are observing here are the "tin whiskers" that can grow from various types of tin-bearing solder, or any application where electromigration becomes strong enough to "push" metal crystals around.

Here's the NASA page on tin whiskers for further reading: https://nepp.nasa.gov/whisker/