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Ion Focus / ZnS Screen

Posted: Fri Nov 30, 2007 5:51 am
by Mark Rowley
Soon I will be testing out my newly designed ion gun. I want to use a ZnS screen to use as a test target. I have plenty of ZnS but am unsure of how to coat it onto a metallic surface. Any ideas?

I was thinking of making a paste of some type but am worried about the potential for outgassing.

Mark Rowley

Re: Ion Focus / ZnS Screen

Posted: Fri Nov 30, 2007 8:29 am
by DaveC
Mark - If you have access to a copy of Fred Rosebury's "Handbook of Electron Tube and Vacuum Techniques." Pub. AIP - One of the American Vacuum Society's Classics.. See the section on Fluorescent Materials (p250 -253 in the 1993 edition) which gives the procedure, I summarize below.

Assuming you are putting the ZnS on a metal screen which can then be grounded for charge removal, then the following steps will give you a reasonably good result. (For nonconducting screens you need to add in Barium Acetate to make a slightly conducting binder.)

The overall procedure is to make a very dilute slurry of the Zns in a solution of Potassium Silicate... (Sodium silicate - water glass also seems to work about the same) . The Silicate solution makes an inorganic binder for the ZnS which keeps it from being blown off by the electron or ion beam.

The key to getting an even dispersion is to put the cleaned and degreased screen base plate on a smaller metal plate (so that you can get under the screen edges, later, to lift it out) - in a large, non-metallic container or beaker (I use a 2000 mL pyrex beaker) filled at least half full with distilled or de-ionized water. Be sure NOT to use softened water as it will have sodium carbonate in it.

The actual amount is not critical in any way. I fill the beaker about 3/4 full. All this water merely forms a cushion which acts to disperse the slurry of screen material.

For the ZnS slurry, you make up enough to essentially cover the entire bottom of the beaker, including the actual screen.

Rosebury cites using 45 gm (!) of ZnS in 24ml of 35% Potassium Silicate, and then diluting this with about 100 to 200 ml of distilled water - to make a 5 inch diameter screen.

Well ZnS must have pretty cheap in his day. I just use a few gms at the most, with the same amount of silicate solution. (I ordered Sodium Silicate from McMaster Carr.. and diluted it appropriately).

So, what you have when you begin the depositing process is a large beaker with about 1000 - 1500 mL of distilled water in it and smaller container with about 200 mL of the slurry.

To lay down the screen, all you do is stirr the slurry so the ZnS is well mixed and then slowly drop it with a pipette into the large beaker. The pipette should be very close to the liquid surface so the slurry does not jet downwards into the cushio liquid.

You probably will need to stir each time you refill the pipette. Drop from different locations, to make an even distribution, and don't hurry. It will go slowly, but in time, you will use up all the slurry, and the cushioning water will be cloudy with the dispersed screen material.

Cover the beaker and let it sit overnight or for a few hours at least, until the particles have all settled. Done properly, you will have a rather uniform screen, at the bottom of almost a half gallon of clear very dilute sodium silicate solution.

The fun comes in decanting all water without disrupting the newly deposited screen. I do it with a siphon.. from the benchtop to a large jar/jug on the floor. Start the siphon and allow it to remove most of the water. A glass tube on the end helps keep the flow away from the screen. You need to do this by hand... so that as the water level gets down to the last few mm, you can stop and remove the rest with a suction bulb or a small plastic disposable mini syringe, if the siphon is too vigorous.

Take your time here... and when there is only water on the screen itself, stop. You will need to let this evaporate on its own. Very gentle heat will accelerate the process... but now is a good time to put a paper towel over the beaker and go do something else for about a half day. You can ruin all your work in about 2 seconds, here with a false move.

Once the standing water is gone... you will need to very gently remove the screen with some sort of grippers... or tongs... and very thoroughly dry the screen.

A heat gun CAN be used from a distance... to keep the air flow gentle... but a slow dry out in a warm vacuum oven is probably better. You can, of course bake it out in a regular home oven..too. Just watch for large air currents. A convection oven is probably not a good choice.. You can eventually bake out at fairly high temperatures. Rosebury cites 350C for a glass CRT.

I use only about 100 to 120 C. Once in the ion beam it will get plenty warm. It can take a few hours once it is under hard vacuum to get the remaininng bound water out of the screen.

But that's the basic process...

Save the removed water and slurry. After it has re-settled, you can throw away all the clear water, and save the concentrated slurry. The Sodium silicate will soldify if the water evaporates. But put in a sealed container, it will stay liquid, and you should be able to re-use it a number of times. Add a bit more of the silicate solution, to bring the slurry back to its original concentration.

Hope this is useful to you...

Dave Cooper

Re: Ion Focus / ZnS Screen

Posted: Fri Nov 30, 2007 5:00 pm
by Jon Rosenstiel
Mark-

http://www.ddc-co.com/publications.html Paper # 39 may be of interest to you.

Jon Rosenstiel

Re: Ion Focus / ZnS Screen

Posted: Fri Nov 30, 2007 8:14 pm
by Wilfried Heil
Dave, a very good description of the sedimentation technique. Thanks for mentioning this book. Such resources are becoming rare nowadays.

If the screen does not need to be transparent, because it is deposited on metal instead of glass, then a thicker coating will be allowed and other methods can be used:

1. Spin coating produces very even layers on a rotating platter. This technique is normally used during silicon wafer production.

2. Spray coating. I used an air brush to make my screens, with very fine powdered P43 phosphor and a drop of NaK silicate as an inorganic binder. Since the silicate is water soluble, you can easily wash it off and recoat the screen, if needed. These layers are stable when handled, but still very sensitive and can't be touched!

The phosphors can be obtained here:
>http://www.phosphor-technology.com/products/crt.htm

Re: Ion Focus / ZnS Screen

Posted: Fri Nov 30, 2007 8:49 pm
by DaveC
Wifried -


Thanks for those notes, particularly on the air brush. I have one, but haven't tried it for screen applications. It could make the whole process take far less time.

For "quick and dirty" screen fab, I have also used a slurry of phosphor in methanol or isopropyl alcohol. with an O ring fastened with a tiny line of silicone grease to a glass or metal plate to define the screen area.

Just stir the slurry, and pour quickly up to the O ring height. Step back, and wait for it to evaporate... no open flames , of course.. Quality of this method is a bit iffy, but if you need something right now... it does the job.

Dave Cooper

Re: Ion Focus / ZnS Screen

Posted: Fri Nov 30, 2007 10:46 pm
by Mark Rowley
Wow! That article is great. Thanks Dave. I just may have to take the $50.00 hit and order the book (That is after I purchase the Bell Jar publications). Anyway, the screen I plan on making is actually quite small. 35 mm to be exact. So the good thing is that I wont have to use barrels of ZnS. As it turns out, I have a good supply of sodium silicate and all the necessary glassware.

Again, thanks Dave and Jon for the resources.

Mark Rowley

Re: Ion Focus / ZnS Screen

Posted: Sat Dec 01, 2007 2:04 am
by Jon Rosenstiel
Dave-
I concur with the others.... excellent post. I have printed it out and filed it.

Jo nRosenstiel

Re: Ion Focus / ZnS Screen

Posted: Sat Dec 01, 2007 8:06 am
by DaveC
Jon - Let me return the thanks , also, for that post of yours. Those refererences were very interesting. I've got them in my favorites list, now for future use.

Dave Cooper

Re: Ion Focus / ZnS Screen

Posted: Sun Dec 02, 2007 8:28 pm
by John Futter
This technique works well to although similar to the already stated the colloidon binds it all together and it does work with ZnS. Lifted off an electron microscope user group the article in full
""To all interested:
As I said I used green phosphor from JEOL, cat. # 423-011. Call and see if this number still holds true. You will also need a dish big enough for the screen and a cover (preferably glass) with a small hole for removal of the liquid.

1. Remove the old coating from the screen by washing in acetone. The cleaned plate must be free from all particles of matter and the surface must be free from blemishes and scratches.

2. Prepare a sufficient volume of 4% (w/v) suspension of phosphor powder in acetone containing about 1% collodion. The total volume must be sufficent to fill selected dish with liquid to a depth of about 1cm above the surface of the screen in position on the bottom of the dish.

3. Agitate the suspension vigorously (I used sonicator) then pour it rapidly into the dish.

4. Wait about 5-10 seconds to allow larger particles to settle and swirling to cease.

5. Slide the plate smoothly into the liquid, preferably without scraping the bottom of the dish.

6. Cover the dish and leave to settle. When the suspension has settled and the remaining liquid is clear, draw off the liquid by inserting a suction tube through the previously prepared hole in the lid of the dish. It is extremely important not ot disturb the screen plate or the liquid above it in any way as this is done. Draw off the liquid steadily and slowly then remove the suction tube.

7. Leave the screen to dry without any disturbance of any kind. Do not lift the lid to inspect the screen until the powder is quite dry because a slight change in drying conditions can produce a visible mark on the damp surface.

8. When the powder is quite dry, remove the screen and wipe off any excess phosphor from the back and sides of the screen.

9. Install.

10. A newly coated screen will outgas for a short time when it is first placed in the microscope. Pumping times may therefore be a little longer than normal at first.

A very small particle size is desirable for high resolution screens and it may be advantageous to agitate the the phosphor suspension in an ultrasonicator before putting it into the dish.

Hope this helps. It can get tricky and you may not get it the first time. Patience really helps!

Phil Rutledge ""

an aside
If you want to view an ion beam without the phosphor we use a quartz plate that naturally fluoreceses an orange /blue hue. it is weak with beams down at 20keV but a handy cam sees it just fine!!

Re: Ion Focus / ZnS Screen

Posted: Mon Dec 03, 2007 4:05 pm
by Richard Hull
All these methods produce superlative screens.

If you just need a crude screen I just use a bit tiny touch of scotch spray adhesive and sprinkle the powder over it to a decent depth on the screen lay a piece of cling wrap over the powder and smash it down with a light weigh, leave for an hour or so and return, remove the cling wrap and tap the large excess of powder off the screen substrate. Then I use a tiny camel's hair brush to brush off the last excess.

The screen produced thusly is NOT uniform and not ideal, but for the average spinthariscope or eye observed scintillator is quite serviceable and quick.

Another solution is to dilute some scotch spray spritzed into a small beaker with a larger volume of acetone and then mix in the powder lay the substrate into the solution and let evaporate.

Screens used for optical extended image focusing as in CRT's of X-ray view screens, etc., MUST use one of the tedious methods described in the preceeding posts.

Richard Hull