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Comment by DrStalker

7 years ago

> Quartz crystals used to be hermetically sealed.

Are they sealed against helium though? It can get through a lot of materials that more common gases can't.

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DrStalker

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blattimwind  7 years ago

Traditional crystals come in what's referred to as a "hermetic metal can", i.e. a metal box soldered closed with leads going through glass seals. I don't know if that's good enough for Helium.

The sealing is mostly because of humidity. I wouldn't expect quartz crytals to be overly sensitive to some gas, since they're basically tuning forks; the crystal physically vibrates in a resonance mode caused by and inducing an electric current across the crystal.

Triesault  7 years ago

Hermetically sealed by definition is airtight, i.e. "excludes the passage of air, oxygen, or other gases". [1] With that said, the diffusion rate depends on both the properties of both the seal and the gas.

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

  • khafra  7 years ago

    Sometimes, definitions aren't the most mechanically sound constructs available.

  • garmaine  7 years ago

    Very few air seals protect against helium, the smallest gas. Most materials, even metals, are like sponges to helium. It'll seep through slowly.

    • Wingman4l7  7 years ago

      I was about to correct you and say that hydrogen is the smallest, but apparently, hydrogen's atomic radius is actually a bit bigger. Helium's radius is smaller due to the larger charge of the nucleus making for a tighter electron cloud.

      3 replies →

    • danburbridge  7 years ago

      Helium can cause problems with the metals containing fusion reactors - He bubbles can get inside the metal structure and weaken it considerably:

      "The sun makes energy by fusing hydrogen atoms, each with one proton, into helium atoms, which contain two protons. Helium is the byproduct of this reaction. Although it does not threaten the environment, it wreaks havoc upon the materials needed to make a fusion reactor.

      "Helium is an element that we don't usually think of as being harmful," said Dr. Michael Demkowicz, associate professor in the Department of Materials Science and Engineering. "It is not toxic and not a greenhouse gas, which is one reason why fusion power is so attractive."

      However, if you force helium inside of a solid material, it bubbles out, much like carbon dioxide bubbles in carbonated water.

      "Literally, you get these helium bubbles inside of the metal that stay there forever because the metal is solid," Demkowicz said. "As you accumulate more and more helium, the bubbles start to link up and destroy the entire material.""

      https://phys.org/news/2017-11-fusion-energy.html