[time-nuts] Quartz resonator aging - Vacuum backfill and getters

[Bob kb8tq]

Hi

Helium is still a pretty good thermal conductor at low pressures. The reason
you backfill is thermal. Neon (or other heavy atom gasses) are relatively poor
thermal conductors. 

Getters are wonderful as long as they don’t outgas. Turns out that is not as
simple a thing to “fix” as you might think. They tend to have an affinity for this 
or for that, but spew out something else. 

One “solution” to the problem is to activate the metal wall of the holder so that 
it acts as a getter. You already have to deal with whatever it might spew ….

Bob

> On May 7, 2019, at 4:25 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> Moin,
> 
> I stumbled over one small question in my quest to understand quartz
> resonator aging (and production). One is, that most people who mention
> backfill of resonator packages talk about Helium backfill. Yet Helium
> has quite high diffusion rate through metall (not as bad as Hydrogen,
> but still noticable). While I doubt it would be a major factor in
> aging, I keep wondering why not Neon? Neon is big enough that it has
> no significant diffusion through metals while still retaining the
> property of being non-reactive (until noble gases become reactive
> enough to talk about, you need to go at least down to Xeon).
> The only reason I could think of is, that Neon is too heavy, which might
> lead to higher damping for the same thermal conductivity (pure guess,
> I didn't do the math, nor did I find any references to this).
> So, can the experts here englighten me, why nobody talks about Neon?
> 
> The second question I have is, why very few people talk about getters?
> So far, I have found only one reference of getters being used in
> resonator packages. With absorption being one of the major aging
> mechanisms, I would have guessed that getters would be a big thing
> in crystal resonator design. Why aren't they?
> 
> 
> 				Attila Kinali
> -- 
> <JaberWorky>	The bad part of Zurich is where the degenerates
>                throw DARK chocolate at you.
> 
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