What was the large-scale commercial procedure for making electrodes that pass through the glass without letting air in? I assume that electronics manufacturers must have been making millions of such vacuum tubes in the past. Is the knowledge lost (or not practical for hobby use)?

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adrian_b 6 hours ago | parent | next [-]

As mentioned in TFA, the most important factor for successfully joining a metal and a glass is to match their thermal expansion coefficients.

Most pure metals have a much greater thermal expansion than any glass, which will cause cracks.

In the nineteenth century, the first successful joinings of metal with glass were done using platinum, but that is obviously too expensive for normal applications.

Eventually a special alloy of iron-nickel-cobalt was developed, which is named kovar and whose thermal expansion is matched to that of a certain type of borosilicate glass.

The use of kovar was widespread in electronics, starting with the vacuum tubes and gas tubes, and then continuing with the first generations of transistors and integrated circuits, which used metal packages.

All the old transistors and operational amplifiers that were packaged in metal cans had pins and package bases made of kovar.

When kovar had to be joined with a different kind of glass than the type with which it is matched in thermal expansion, that glass was coated in one or more layers of different kinds of glasses, with that matched to kovar in contact with the metal and the intermediate layers having intermediate thermal expansion coefficients, interpolating between the bulk glass and kovar.

Kovar is not a good thermal or electrical conductor, which is why the modern power transistors that use plastic packages (e.g. TO-247) and copper bases and pins (which are plated with nickel or tin, to avoid corrosion) can easily dissipate much greater powers than the old transistors in TO-3 metal cans, which had the same size. On the other hand, the old transistors in metal packages were pretty much immune of environmental influences.

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SoftTalker 6 hours ago | parent [-]

Ordinary incandescent bulbs must have similar sealing requirements, but they probably mostly rely on using a thin conductor that doesn't contract much when it cools. Also IIRC modern incandescent bulbs do not use a high vacuum but contain a low pressure inert gas so leakage would be slower if it occurs at all.

	▲	adrian_b 5 hours ago \| parent \| next [-]
		As you say, incandescent bulbs are less demanding because they do not use high vacuum, but they have the additional requirement that the pins that support the tungsten filament must resist to very high temperatures, because some heat is conducted through the filament into its support. This is why the pins that support the filament are typically made of molybdenum. Molybdenum has a relatively low thermal expansion coefficient in comparison with most metals, so there are certain glass compositions that can match its TCE. The glass through which the pins pass is not of the same type as the bulb, which is made of cheaper glass, but it is of the type matched in TCE with molybdenum.
	▲	sam1714 18 minutes ago \| parent \| prev \| next [-]
		Incandescent bulbs are 0.8 atm (equivalent to 15,000 ft altitude) when off, and when they're on, they're actually slightly pressurized.
	▲	ssl-3 5 hours ago \| parent \| prev \| next [-]
		AFAIK, in in the US: Modern high-efficiency incandescent bulbs are exactly like the halogen bulbs of yore, but with the small quartz envelope wrapped in a familiar light-bulb-shaped shell, and with a base that matches. It's like a light bulb within a light bulb. See this random example of a GE bulb (which I selected just because it includes the first picture I could find of a modern bulb made with clear glass): https://www.toolboxsupply.com/products/ge-lighting-62616-ene... Except for all the ones that aren't modern or efficient. Common 40-Watt appliance bulbs, for instance: Those are still built using the old methods. They never changed. This strongly suggests that we never forgot how to seal metal wires into a glass bottle full of nothing. But this article isn't about industrial processes. It's about rediscovering things at home, and that stands on its own merits. :)
	▲	tliltocatl 5 hours ago \| parent \| prev [-]
		Incandescent bulbs used dumet/platinite, which is an nickel-iron alloy like kovar except it's turned for a different CTE. This stuff isn't that expensive when mass produced - it's just those who can afford usually borosilicate can afford paying a premium for kovar.

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SoftTalker 6 hours ago | parent | prev | next [-]

Vacuum tubes are still made today, so I'm sure the knowledge is not lost. I'm curious about the answer as well.

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NoMoreNicksLeft 6 hours ago | parent [-]

I was under the impression that they were only made in eastern Europe at this point, former bloc nations. Even then, the demand must be microscopic at this point.

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musictubes an hour ago | parent | next [-]

They are also made in China. There is also one American company that makes a single audio tube, the 300b. They bought the Western Electric tooling, materials, and trade dress from whoever had bought it from AT&T.

I’m not sure where non audio vacuum tubes are made. I’m sure there’s a variety of companies around the world making X-ray, transmitter, laser, etc. tubes.

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drum55 5 hours ago | parent | prev [-]

xray, photomultiplier and laser tubes are still SOTA. PM tubes in particular have a huge number of glass feed through for the intermediate plate voltages.

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SoftTalker 4 hours ago | parent [-]

Musical instrument amplifiers still use them as well, at least some of them.

	▲	namibj 2 hours ago \| parent [-]
		Those are Not where the tubes are SOTA.

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ludicrousdispla 6 hours ago | parent | prev | next [-]

I'm not sure what specific glass and metal are used in neon sign electrodes, but their definitely built to hold a higher vacuum under decades of use. Their relatively cheap and you can get them with small tubes on the end for pulling the vacuum.

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CamperBob2 6 hours ago | parent | prev [-]

The article pretty much tells you: "Copper's red oxide bonds very well to glass. In fact, the bond is stronger than the bulk glass: when it breaks, there's always a thin layer of glass left stuck to the metal. Along with its excellent electrical properties, it seems like an ideal electrode material." If you look at how vacuum tubes are constructed that's essentially what you see.

Tubes are evacuated through a hole created elsewhere, nowhere near any electrical connections. The getter is then flashed to clean up any gas molecules left over.

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adrian_b 6 hours ago | parent [-]

Nope, as also mentioned in TFA, copper has a too great thermal expansion coefficient in comparison with glass.

If vacuum tubes had pins of copper, the glass-metal joining would have cracked very soon during normal usage cycles, and there would have been no vacuum left in the tube.

Real vacuum tubes and gas tubes had pins made of kovar, which is a Fe-Ni-Co alloy with a TCE matched to a certain composition of borosilicate glass.

The kovar pins were normally plated with nickel on their external parts, to enable soldering, because molten solder does not wet kovar.

	▲	CamperBob2 6 hours ago \| parent [-]
		Right, point being that metal-glass seals are very effective given the right materials. This has nothing to do with how the tube is evacuated, and there is never a point where the wires have to be "passed through the glass without letting air in."