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In Reply to: RE: Anyone used an induction heater to re-activate the getter? posted by AmadeusMozart on February 15, 2021 at 15:48:18
I'd be curious....How hot? How Long? And in what atmosphere.....I'd suspect something fairly inert, like perhaps Argon.
Too much is never enough
Follow Ups:
Excerpt: (the document can be found using your friend google)There are some 'fixes' for low-emission and poor heater-to-cathode resistance that will be covered later in this
article, but ten valves with the same symptoms as the faulty D3a could now be sacrificed to science with impunity.
Since chemical reactions double their rate for every 10°C rise in temperature, perhaps the getter could be provoked
into restoring the vacuum by heating the valves in a kitchen oven? Maximum envelope temperature is typically
specified as 200°C, but kitchen ovens are hardly precision devices, so the author's oven was set to 'warm' (which
turned out to be roughly 100?C or 212?F). Three hours later, the valves were removed and re-tested. Gratifyingly,
there was a noticeable improvement, with all ten valves registering approximately half their previous gas current.
Clearly, the hypothesis was plausible, but the process was rather slow and it was a hot summer, so leaving the oven
on for a long time was not attractive. A back-of-an-envelope calculation suggested that reducing the residual gas
and gas current to <1% of its original value would require more than 7 x 3 = 21 hours (27 = 128). A cookery book
suggested that gas Mark 2 is around 130°C (266°F), and as the oven temperature had previously been measured
with a thermocouple probe as being approximately 100°C (212°F), this should give an eight-fold improvement and
reduce the time required to 7 hours.
Although the AVO valve tester was capable of indicating the gas current and its improvement, it could not make
an accurate measurement, so it was modified by breaking its grid bias supply and connecting a Fluke 89 IV -
ammeter in series to enable a better reading.
The D3a valves were added to the EF184, the oven was set to gas Mark 2 - later measured to be 120°C (248°F) -
and the valves were left overnight for 13 hours. On removal, the valves were tested for gas current, and the results
after baking were added to Table 1.
Although the gas current measurement before baking was necessarily somewhat inaccurate - 1µA on a 100µA FSD
movement - the average improvement in gas current due to baking was a factor of five. Testing the baked valves
showed that although anode current and gm were noticeably lower for all valves, they were far more stable.
Further, they now agreed closely with a known good valve, suggesting that the previously high and unstable
characteristics were a direct consequence of the grid gas current.
Although the baked valves showed an improvement in gas current, the very soft valves remained low emission and
were set aside.
Baking tips
Valves that have no manufacturing defects but have been in storage for many years may accumulate a little gas. If
there is any suspicion that this may have occurred to any significant degree, it is better to bake the valves before
testing rather than risk damaging fragile oxide-coated cathodes.
Grid ionization current can easily be the dominant form of noise in high resistance circuits, such as condenser
microphone head amplifiers. As a result, it makes sense to routinely bake valves intended for this type of use
before selecting for low noise.
Although valves with glass button bases may be safely baked at 120°C (248°F) for 12 hours without damage,
baking valves with phenolic bases at this temperature causes the surface of the base to erupt.
This was not a welcome discovery...
Edits: 02/15/21
Great information. And has enough convincing data.
I learned how to run a vacuum system, quite similar to the one shown at the link. Later systems had automatic valve sequencing so the rough value and hi-vac value would not be open at the same time.
Most systems also includ a Cold Trap on TOP of the diffusion pump to trap any volitiles going either way. The pumping media of a diffusion pump is a VERY high grade of Silicon Oil. The cold trap is typically full of Liquid Nitrogen which condenses LOTS of bad stuff.
When doing system service, You'd close the hi-vac value and let the system pump. Then 'blow out' the cold trap using nitrogen. That warms it up. And you'd see a huge pressure burst in the foreline as the stuff trapped on the cold trap evaporated and was pumped out. Forelline pressures over 100 microns were common at that point.
The best systems would be to the mid to bottom of the '7 range' while a PERFECTLY clean system which had been baked (internal heaters) THAN having the cold trap filled could JUST bump into the 8 range.
Too much is never enough
In my younger years I did built a few reflector telescopes and made my own vacuum coater for the telescope mirrors. Had to get some deep vacuum otherwise the reflective coating would just come off. Learned the value of slowly heating the tungsten filament. The things we figured out before the days of the internet or satelites, far more satisfying and a slower pace of life to boot. But one could not get the quality parts to build a tube amplifier these days without the web so not all is bad.
AM
How good a vacuum could you achieve and how did you measure it?
What kind of pump? 'Mechinical' only, either piston or rotary vane will probably not get you a good enough vacuum for metalization of a mirror.
I coated LOTS of Silicon wafers. Front AND back. Frontside coatings are or Were a 6000 series Aluminum alloy. That would be Silicon at a maximum %age to prevent precipitation and other bad stuff when doing the sinter / alloy step near the end of hte process.
Backside coatings were either a proprietary layering ending with Silver or simplly Gold.
Some metals dont stick well to others and surface preparation is critical. Some films will 'shrink' or 'stretch' after depostion resulting in FILM STRESS measured by the bow of the product. Such films will be either Tensile or Compressive....In extreme cases stuff will simply 'peel' off. Control of depostion parameters helps. Temperature? Rate of deposit? Ultimate thickness? And a bunch of other stuff.
I'll skip the gorey details, but the 2 main techniques of applying such a film are either Sputter or Evaportiaon. Sputter is done in an Inert 'backfill' atmosphere, usually Argon. But Still and all? Depostion presssures in teh 5-range are necessary.
Too much is never enough
I used a two stage mechanical rotary pump and that followed by a diffusion pump. Largest mirror I could do was 10".I had left Philips by that stage but I knew the purchasing officer and I knew that sometimes perfect new tools were scrapped. So I just rang up the hthe guy in charge of destroying stuff in the scrapyard and mentioned that I had heard from the head of purchase that there was a diffusion pump floating around to be scrapped but that I needed it. I picked it up for scrap metal price. Same with the rotary pump. Brother was an engineer and he made a 20mm thick stainless steel bottom plate holding the diffusion pump with some feedthroughs for holding the tungsten filament.
I did not have any measuring equipement but a friend of mine told me to wait for the air to get cloudy and then wait so long before switching on the diffusion pump (forgot how long) and then pump for another whatever time he gave me. I did have a meter on the rotorary pump but have by now forgotten all the details. (That friend also ground an 1 meter experimental mirror for NASA.)
In the end I realised that I was extreemly lucky that nothing untowards happened as I did not have any protection around the glass. It would have been fatal if it had imploded. I only coated pure aluminum for reflection.
(Originally I used a chemical process for depositing silver but never found it satisfactory).
Edits: 02/25/21 02/25/21
You could have improved your vacuum by about an order of magnitude AND made it cleaner inside had you a Cold Trap OVER the diffusion pump. You 'd have needed a source of Liquid Nitrogen and a 'thimble'....
That would prevent backstreaming of the really nasty Silicon Oil (DC 702, for example), or maybe Fomblin? But that might increase the complexity of the valving.
My prize system, a CHA had a Roots blower on top of a mechanical pump. This thing would go to 50 to 100 microns in about 1 minute and cross over where it would bury the pressure meter and immediately drop into the 6 range. This was a fairly open chamber design so the mean free path was large. The High Vac valve was maybe 18" in diameter. A real monster, as such things go. After a service once where I cleaned it thoroughly I got it to the bottom of the 7 range before flashing with aluminum to 'passivate' the interior.
I keep wanting to get involved with Palomar which re-metzlizes the 200" mirror every couple years. I know they don't have a good setup. I've seen their work and plugging leakss with Apezion L is not the way to go.
The problem doing it your way was outgassing of the DP as it warmed than waiting a LONG time for it to cool.
More elaborate valving would have helped at substantial additional expense.
I finished my metalization days with sputter systems. Varian made systems for than common 5" and 6" silicon wafers. No DP on those guys since they'd gone Cryo by that point.
Too much is never enough
The object at the time was to be able to maintain without having to rely on someone else who might clean the mirror of the old aluminium reflective layer and in the process damage the surface. As such it was not important to me how long the process would take and the trick was to invest as little as possible: all up I spend less than 10% of buying through normal channels all the parts. Most of the costs were for buying a new glass dome, seals, oils and a variac for controlling the filament.
The mirror was made from zerodur which came from Zeiss in Jena. Darn hard stuff.
AM
When I worked making low frequency quartz oscillators, we had an optical flat which I think was Zerodur. 1/4 wqve? 1/8 wave? We used it as a reference to maintain grinding tables for making quartz (harder than the law allows) wafers for the manufacture of tiny tuning forks....
Aluminum is prone to oxidation. In a more sophisticated process, you might want to, and without breaking vacuum, apply a very thin coat of Silicon as an anti-reflective coating. Or something else to passivate the surface. I don't know common practice in telescope mirrors.
The OLD mirror can be taken off the substrate using something like Phosphoric Acid or another aluminum etch. Maybe Potasium Hydroxide? NO abrasives or anything else touching the surface.
the layer shouldn't be more than a micron or so thick, anyway.
If you are within driving range of me, I'd love to come watch.....
Than if possible, heat the substrate in a decent vacuum (DP not needed in this case) in order to outgas all the moisture.
Too much is never enough
besides having to travel to New Zealand (where I live) you would need to do also time travel to 1981 as this stuff has long agao disappeared....
Just when I ran out of time machine frequent flier miles!
BTW? Another way to get hi vacuum is to use a Turbo molecular pump. Basically a VERY High Speed vaned turbine. An air molecule 'wanders in' (at very low pressures, random or molecular flow rules) and gets 'batted' towards the exit.
VERY clean but low tolerance for injestion of larger particles. A littel $$, too, and needs an external controller / PS.
Too much is never enough
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