Why is it better to hermetically seal quartz crystal oscillators in helium gas rather than nitrogen gas?

Answer 1

Sorry, John S, but you are absolutely wrong: helium is the LEAST reactive element -- it combines with nothing. But that is beside the point here. The reason for using helium would be to help stabilize the temperature, and hence the frequency, of the oscillator. The crystal in question is probably in a constant temperature oven. The other plausible fill for a crystal container is vacuum: the temperature control is not as good, but because the crystal does not lose energy to the surrounding atmosphere, its Q is higher and hence its frequency more stable. This is the kind to get if you aren't using an oven (such as in a portable device, where the power is limited).

Answer 2

I've not heard of helium being used.

Vibrationally, there are lots of different types of quartz oscillators. There are shear mode (gas type would not matter here), thickness mode and bending mode.

Bending mode oscillators tend to be lower frequency, e.g. 50 kHz. In this case it is possible to setup acoustic standing waves inside the device case. With helium, the wavelength is tripled compared to air. If the case dimension is smaller than 1/2 of a wavelength, there should not be any strong resonances that might otherwise interfere with quartz resonance.

Answer 3

I'm not familiar with the specific application, but helium, despite what John S says, is quite inert (it's a noble gas). Also, helium is commonly used as a leak indicator in hermetically sealed instruments, so the helium fill provides a means of verifying seal integrity.

Answer 4

Well, I don't know. But I can make an educated guess: Helium is chemically inert. It doesn't react to anything under most conditions. Nitrogen isn't chemically inert. So if you seal it in Nitrogen, there may be something in the quartz that can slowly react with the nitrogen and mess up your crystal.

That would be my guess.

Answer 5

to inquisit... who answered this question before me, get your money back for your physics degree. helium is one of the most reactive elements known.