3 ms·

Depends on the co-efficient of expansion of the metals and the physical strength of the glass. Metal/glass insulators have been a thing for 100+ years, for exa

• adrian_b 6mo ago
  True, but those metal/glass devices are made by joining a pair of a certain metal alloy with a certain kind of glass, which are chosen to have matched thermal expansion, for instance kovar alloy with borosilicate glass. If you join a random metal-glass combination, it will fail soon. When incompatible metals and glasses must be joined, the metal is covered with an alloy and the glass with another kind of glass, so that at the interface you have a matched pair. Sometimes more layers with different kinds of glasses are used, to ensure a gradual change in the coefficient of thermal expansion, so that no adjacent glasses would differ too much, which would cause cracking when the temperature varies. In TFA it is discussed that the problem is that copper has a much higher thermal expansion than any glass, so you cannot match it. Moreover, at the very small sizes that are desired, it is hard to cover copper with some other metal that is matched with glass, without leaving too little volume for the copper, which would increase the electrical resistance. For the manufacturing of electronic vacuum tubes and also for the first transistors and integrated circuits, which were packaged in metal cans, the discovery of the kovar alloy, which can match the thermal expansion of a certain glass, was a necessary precondition. Kovar was a derivative of the invar alloy, which has a much lower thermal expansion than most metals.