[Milsurplus] Question for Metallurgically-Smart People

[Bob kb8tq]

Hi

If you want to simply remove the metal (but not impact the quartz), Chromic 
Acid (rather than a fluoride compound) is the preferred approach. It’s sort of an
all or nothing kind of thing. A partially removed metallization is going to be a bit
flakey (both literally and figuratively). Just as with the pencil, it is difficult to get
uniform metal distribution when dissolving it. 

The other reason the “pencil” approach sometimes fails is the required bonding
of the material to the blank. The previously mentioned “WWII problem with 
crystals” had at it’s heart, very small amounts of debris left on the blanks from
the manufacturing process. They didn’t fail immediately, but died in the field. 

That same issue makes any metal deposition scheme that is not very good a
problem long term ….

Bob

> On May 22, 2020, at 8:23 AM, David Stinson <arc5 at ix.netcom.com> wrote:
> 
> Thank you all for the lively discussion.
> The reason I ask:
> Off-and-on, I've been working with crystals, etching them to freq
> or "pencilling" them down as needed.
> Some crystals have a silver plating on the blank.  This isn't for
> connectivity; it's to add mass and inertia to the blank in order
> to bring the crystals oscillation frequency down.
> 
> I discovered this when trying to etch these crystals.
> The blanks were in "regular" holders; i.e. between two
> metal plate with raised corners- no direct contact
> with the metal coating.
> In the etchant, the metal coating bubbled vigorously.
> To my surprise, etching that metal coating moved the
> crystal's frequency up rapidly and by a large amount.
> I didn't make notes at the time, but I estimate I etched it
> for a couple of minutes, then became concerned at how
> quickly the metal was dissolving.  I removed the blank,
> cleaned it and tested, finding the crystal had moved a
> huge amount- I didn't write it down, but it was tens
> of KCs!  Way past my intention  IIRC, the blank started
> at something below the AM window on 80 meters and,
> after the short etching, ended above 4 MC.
> That was hard to imagine, so I tried another on a
> useless freq and the result was the same.  I did manage
> the use some of these crystals, but the etching time
> was in seconds!
> 
> This is the same principle as "pencilling-down" a crystal,
> but with the uniformity and fine control needed to get
> the crystal spot-on and still active.  One reason "pencilling-
> down" often fails is because the random distribution of
> material on the face of the blank can cause it to stop
> ringing properly.  The metal coating is uniform, so it adds
> to the inertial moment of the blank without the random
> irregularities of "pencilling" that can kill oscillation.
> If there were a practical way to deposit this material
> on crystal blanks, there's just a world of now-useless crystals,
> too high in frequency, which could be "re-tuned" to usefulness.
> 
> If I live long enough for another project,
> I'll play around with this some more.
> Be a while- still have parcels to ship and
> radios begging for attention ;-).
> 
>  GL OM ES 73 DE Dave AB5S
> 
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