When quartz pinch seals are made, the seal is typically made with molybdenum foils going through the quartz. I have the following questions about this process:

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1) Why Molybdenum?
Quartz has a CTE of around .55e-6 which is very very low. So you would want to use the metal with the lowest CTE that you can find, right? Well (according to Google AI overview) molybdenum has a CTE of 5.2e-6 while tungsten has a CTE of 4.8e-6. Why wouldn't they choose tungsten for this application?

2) Definition of "Foil"...
So they use molybdenum foil for these seals, but what exactly do they mean by foil? Are we talking about .1mm or something like .01mm? I would assume the thinner the better, but obviously there is a practical limit to the thinness of foil.

3) "Feathered Edges"?
I always hear descriptions of Mo foil seals mentioning that the edges of the foil are "feathered". What is that supposed to mean? When I look at a halogen capsule all I see is foil, I don't see any visible edge treatment done to the foil.

4) Vycor Glass?
I just learned that there is something called Vycor, which is 96% quartz, but technically a type of borosilicate glass. It has a CTE of ~.75e-6. Was this ever used in lamps? I feel like straight up quartz might be overkill for something like a small halogen capsule.

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Thanks!

You need something that has a reasonable adhesion to the quartz/glass in order to form a good seal, it needs to be workable into rather thin foil, it should be weldable, withstand working temperatures when forming the seal and withstand temperatures needed to form the seals. Plus it should not be prohibitively expensive material. Plus it should not be that hard, it should allow (preferably elastic) deformation without needing that high pressures

The cross section needs to have high aspect ratio between width to thickness. Because the trick is to redirect the expansion into the foil thickening, instead of widening, without creating excessive forces to crack the quartz. So then the quartz walls just have to slightly flex out, to create the room for the material expansion. And the thinner the foil, the less the quartz has to flex, so the tougher seal you can get.

"Feathered edges" are there to hold it in place, so the forces related to expansion won't move it and by that lose the adhesion and form possible leak paths. Plus because the thickness is never exactly even, any shift would mean stress concentrations on some spots, sp making it more likely to crack. usually normal burrs from the foil cutting are enough, it means the tooling is explicitely designed to keep them in place and not eliminate them (as is the aim for any normal sheet/foil metal processing).

Gradual seals (so gradually transitioning from one expansion coefficient to another to distribute the expansion) is a technique that was (and I guess sometimes even is) used in lamp making. But it is quite complex process, a lot of steps, so not that much compatible with high volume production. So generally it is avoided whenever possible. And with quartz it is possible...

I believe also that feathered ends serve to discourage hairline bubbles forming along the sides of the foil, which cause slow leaks, a nightmare for the manufacturer.

@Medved
So tungsten could be used, but it is just impractical?

So "feathered" just means it has really sharp cut edges, right?

I was also aware of those graded seals to tungsten wire, but like you said they are barely ever used anymore.

Tungsten is very hard metal and that is what makes it not practical I guess...

Feathered means sharp edges, as well with metal spikes drawn from those edges. The kind of edge you get if you want to cut a sheet metal with shears but the shears are very of bad quality.

@Medved
Ahh I see, good to know. I guess maybe tungsten would be too fragile in foil state.

Correct for all except the feather edged part.  Once the Mo foil has been rolled down to the desired thickness, it is changed from a rectangular to an eye-shaped cross section.  It is thicker at its centre than its lateral edges.  This special cross-section can be achieved either by rolling (the cheap way) or by electrochemical etching (slow but makes much better seals).

The purpose of sharpening the foil sides to knife-edges is to reduce its strength as well as to attain better all-round adhesion of the quartz.  Without this there tend to be airlines along the edges of the foil which can cause lamp leakage.  Moreover as the foil expands laterally when the lamp warms up, it of course puts far greater stress on the quartz than when its thickness expands.  The feathered edges are so much thinner than the centre of the foil that they are able to deform elastically and absorb that expansion without creating cracks.

You might well then ask, why do we only feather-edge two sides of the foil rather than all four.  That is only for cost reasons.  In most lamps that have smallish foils and low currents, it is sufficient to feather only the longitudinal edges to guarantee a gas-tight seal.  The other two edges of the foil don’t stick to the quartz anyway because there is a lead wire, filament or electrode welded to it, along with a surrounding air gap to ensure that the expansion of those parts does not crack the quartz.  Only in higher value lamps, or lamps with wide foils carrying more than 10-20A do we feather all the edges.  That is slow and expensive, because the foil is usually supplied on continuous reels of something like 10-50m length.  Due to the nature of that continuous ribbon only the two longitudinal edges can be feathered.  After cutting down to the length needed for a lamp, the individual pieces have to be picked up in tweezers and immersed in an electro-etching solution to sharpen the edges all around.

Following the invention of Vycor by Corning, it was of course instantly studied by all lamp makers.  It used to be far cheaper than quartz, and has the advantage of slightly lower softening temperature which allowed production machinery to run faster.  It is however less pure than quartz and that interferes with the chemical transport processes in many lamps.  It cannot be used for metal halide.  In halogen it can be tolerated for short-life projection types only.  I believeSylvania was first to launch a Vycor lamp in the form of the DYS single ended halogen capsule for overhead projectors, but some evidence suggests it may have been GE.  This realised a massive cost reduction not only because the material is cheaper, but also because the tubing used for the bulb could be halved in diameter.  The narrow Vycor tubes were first heated and inflated to form a larger diameter spherical bulb around the big 600W filaments, quickly and at low cost due to the rapid heating yo softening temperature.  Then the narrow diameter neck could be pinch-sealed faster than usual due to its reduced mass of material to be heated. 

The only other major breakthrough of vycor was for the exhaust tubes of halogen lamps, even many long-life types.  The remaining volume after tipping-of was small enough that the extra impurities of vycor could be tolerated.  The main advantage here was once again cost, and that the lamp exhaust machines could be speeded up due to the faster tipping-off.  Halogen tipping tends to be slow because the lamps and often even the whole exhaust tube are immersed under liquid nitrogen during tipping, and there is less work for the fires to do if the exhaust tube melts at a lower temperature.

@James
Thanks again for this info!

So feathering the edges of molybdenum foil is not exactly the easiest thing to do, or so it seems.

I remember seeing a listing for a bulb-shaped 600w halogen projection lamp on Ebay a while back (I forget what brand it was), but I remember thinking that would be a lot of work to shape the quartz like that. I guess it may have been a Vycor lamp instead, which makes more sense.

Why is liquid nitrogen involved in the production of halogen lamps? Is it because they have to seal the lamps under what would normally be above atmospheric pressure, so they cool it off to get it under some amount of vacuum?

Halogen lamps are sealed under liquid nitrogen (or sometimes sprayed with it) to attain higher gas fill pressures - which then deliver higher efficacy and/or longer life.  The krypton or xenon gas is admitted into the bulb, then liquefied and the bulb tipped off.  As it warms up to room temperature the krypton in the bulb evaporates and typically achieves cold pressures of around 5-15 atmospheres.  The lamp could not just be filled with such high pressure gas otherwise when melting over the exhaust tube the positive internal pressure would blow a hole in its side.

@James - So why that (5-15bar pressure) is never mentioned in lamp use precautions? An agreement not to scare domestic users? ) Even some small 100W xenon arc lamps likely filled to comparable pressure come with some warning words in the datasheet...

BTW I never thought about halogens as a usable source of xenon gas. If so, some lamps crushed in a pumped - out cylinder may give some valuable amount of xenon for lamp experiments. Somewhat polluted with iodine/bromine, though...

@James
That makes plenty of sense, thanks for explaining