[ARC5] SCR-274 calibration Xtals

[Bruce Long]

Crystal acid anti-aging treatment.
Richard, your explanation is good.  Having worked as an employee or a consulate for Piezo, McCoy Bliley and others I would like to make some comments.
I don't think the upwards frequency drift is a result of the release of entrapped grinding grit.   Somewhere years ago in a public document I saw a pair of electron microscope images of a quartz crystal bank before and after the final HF acid etch.  Even when polished to an optical quality finish, the surface of the blank when viewed under high magnification in an electron microscope shows a surface that looks like a bowel corn flakes smashed flat-  very irregular with lots of little dangle-ly  bits.  

The hydrogen fluoride final etch removes the dangle-ly bits and generally smooths the surface at the microscopic level.
During operation a typical quartz crystal blank physically moves back and forth by about one crystal atomic layer of distance.  Not much at all,  However velocity is the time derivative of displacement and acceleration is the double time derivative of displacement.  In the frequency domain acceleration is proportional to (2 * pi* frequency)^2.  From F=ma  the force acting to tear loose the dangle-ly bits is proportional to operating frequency^2  


This means entirely ordinary, pretty good crystals have surface accelerations well in excess of one million Gs.  That is why it is important to get rid of the dangle-ly bits if you want low aging.


Another interesting, relevant tidbit is that it you take an ordinary 10 MHz fundamental mode crystal and you remove or add one crystal lattice layer of SiO2 molecule, the frequency will change by the relatively hudge amount of 60 ppm-  from John Vig's Crystal Resonantor tutorial.
When a 70 year old crystal moves upwards by .5 ppm over its lifetime we are talking about in average the loss of about one out of every 120 surface Silicon dioxide molecules, a truely atomically small quantity.
bruce  kj3z



      From: Richard Knoppow <1oldlens1 at ix.netcom.com>
 To: arc5 at mailman.qth.net 
 Sent: Friday, August 19, 2016 5:50 PM
 Subject: Re: [ARC5] SCR-274 calibration Xtals
   
    There is a good history of quartz crystal manufacture and its 
expansion during WW-2 at:

http://ihanatech.com/technical_notice/a%20histrory%20of%20the%20quartz%20crystal%20industry%20in%20usa.pdf
    A google search for "virgil bottom" will find much more.

On 8/19/2016 1:47 PM, Dennis Monticelli wrote:
> FYI.  I have worked with surplus WW2 crystals a lot and can say that it
> was relatively common for the upward drift to occur.
>
>  It was not until approximately late 1943 or well into 1944 that ground
> crystals were put through the proprietary Bliley final etch process.
> This process was developed to improve frequency accuracy but
> incidentally cured serious drift problems with early rocks put in
> service in the ET.  The Signal Corps eventually forced Bliley to pass
> that secretive process to other suppliers, though I don't think all
> makers applied it.
>
> How it works:
> Grinding produces lots of fine grit, some of which lodges in the
> roughened surface of the blank and does not wash out.  As the crystal
> vibrates in operation it works loose some of the grit, which reduces
> loading and moves the crystal freq upward.  The final short HF bath
> easily removes the embedded grit along with molecules that are weakly
> chemically attached to the surface.  What is left behind is a well
> ordered stable crystal surface.
>
> An experiment you can do:
> Take a rock from 1943 or earlier and give it a dip in a weak acid for a
> minute, wash thoroughly with distilled water and alcohol, and then
> remeasure freq.  It will move upward.  Do this again for an additional
> minute, then a minute more, etc.  You will observe a curve of freq
> change vs time that drops sharply at first and then settles into a
> linear relationship of slow change vs time.  You are observing the acid
> attacking the "low hanging fruit" and then hitting the well ordered
> crystal.  Usually, activity will improve as well.
>
> Maybe more than you wanted to know :-)
>
    Its never too much:-)
> Dennis AE6C
>


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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