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.

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

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.

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.

> "good enough"

Probably not. The classic fix is a "getter".[1]

[1] https://en.wikipedia.org/wiki/Getter

> Is this really "good enough" for a triode?

Let us not overlook that its also a lot of energy. Its not a matter of "good enough", I think in this case - more "can I?" ..

Plastic generally isn't an air tight seal. The leaks may be slow, but generally we hope vacuum tubes last for years.

An accidental crack can be resealed, but if the crack had appeared because inappropriate materials were used, e.g. an unsuitable metal-glass pair, resealing is pointless, because cracks will appear again after the device is turned on and off several times, causing expansion-contraction cycles.

If you want a gas discharge tube and not vacuum, you can even drop the coil on the inside: https://hackaday.io/project/194683-plasma-toroid-sky-guided-...

But most hollow-state devices run on either DC or pulses, so coupled inductors wouldn't work.

That depends. Often vacuum tubes are used with DC (that is a rectifier) in some form though, in which case you can't do this since induction depends on AC. I'm not sure what purpose the article had for a triode though, depending on their application this might work.

Interesting idea! Wouldn’t have to be particularly high voltage either.