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Author: Subject: Cesium from CsCl
MyNameIsUnnecessarilyLong
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[*] posted on 5-6-2012 at 23:34


Does anyone know how reactive cesium vapor is to copper metal? What about cast iron?


I wanted a wide opening on my chamber for easier loading and cleaning, so instead of using threaded fittings I think I'll just make some knife edge vacuum flanges to couple a condenser assembly to the reaction chamber and have it seal against a copper gasket.

If copper isn't compatible enough then I guess I could make the flanges to be a taper seal and use a soft iron ring instead... But would it be able to deform enough to create a tight vacuum seal?
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[*] posted on 6-6-2012 at 06:46


Copper doesn't react with cesium, and although cesium will likely erode copper over time, attack is slow enough.

Cast iron will also stand up to cesium. Cesium probably will pull certain trace elements out of the iron, hopefully these should (mostly) remain behind during distillation.

All in all, Cu is by far the better choice. Even then, I expect the seal to give you fits if you actually test it out. You should strive for 0.001 mm Hg for the best results. Even though this process can even be done at 10 mm Hg, a good vacuum seems to give a cleaner product as judged by the amount it wets glass. A better vacuum also makes the necessary distillation temperature 100 - 200 degrees C lower.

Pipe threads are actually kind of poor at holding a good vacuum without thread sealant, and few sealants are good at these temperatures. Your move to escape these damned leaky little grooves is a really good idea. The devil, of course, is in the details.

I can't explain it but, in the cases where I collected fractions, the later fractions also appear cleaner (by the same criterion as above). I would have expected the opposite based on the likely contaminants being Li as well as perhaps Rb from the CsCl and Na in the Li. The likely contaminants should be less volatile than Cs, I would have guessed.

In my experience, loading and cleaning are easily done through a 1/2 to 3/4 inch opening. Cleanup is especially easy since everything dissolves in water with varying degrees of excitement.

Remember to put a small wad of steel wool (SS is better still) at the top of the reactor to prevent physical bumping from carrying impurities into the condenser tube.




[Edited on 6-6-2012 by Zan Divine]




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[*] posted on 8-6-2012 at 20:19


This is the general design at the moment based off what materials I already have on hand:





I'm thinking the best way to avoid cesium getting stuck within the coupled area will be to turn a chamfer inside until the radius almost meets the copper ring. Then for the upper flange I'll give it a chamfer and then weld a shroud/flare around that which can nest virtually metal-to-metal against the lower chamfer so that when the whole thing is brought up to temp the thermal expansion can swage this cone against the chamfer. This should, hopefully, both funnel the cesium vapor into the condenser and also stop any liquid cesium or cesium vapor to come in contact with the copper gasket.

The extra space between the chamfers should also help contain any bumped material






And here's what I have so far:




The heating filament needs to go since it's not fitting inside the heat shield/stand. I wanted to use a bendable heating element that hardens to shape after first use, but there's only about 3/16th - 1/8th clearance for the last 1/2" of the chamber between the heat shield. The smallest dia element I've found is about 1/4". Will probably end up using some of that alumel wire or high temp heating tape





This is the Welch 1376 style vacuum pump I'll be using:



This beast weighs ~150lbs! :mad:
Way too heavy IMO when it only pulls down to 1 x 10-4 torr.

[Edited on 6-9-2012 by MyNameIsUnnecessarilyLong]
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[*] posted on 9-6-2012 at 08:35


Verry nice! Good welding skills, too.

With your very good vacuum, you'll possibly be able to get down to the region where flexible heating tapes will work but I strongly advise against it. They were never meant for this temp. regime.

Any of the resistance wires can operate here, the insulation is the trick. Fish spline beads (also as fish spine beads) are sold at numerous outlets. Below is a picture of the 4.3 mm OD beads and the 0.8 mm Alumel wire I used. This is the ideal answer to the need to wrap flexible heat around a metal tube.

The metal shroud around the outside of the heating unit is beautiful but may be counter-productive. It won't reflect much heat inward but will conduct much heat outward (even if it's SS). Do yourself a favor and buy some of the bio-soluble ceramic fibre board in the low temp version. You can cut it with a knife, glue it with furnace cement and it is perfect for this application.

Your initial distillate will wet glass almost certainly. You'll need to redistill. See my earlier posts for literature preps & purification if you haven't yet.

Don't forget the argon. Welding gas, as you know, can be up to 1% O2. Use UHP if you can. If I blow my welding grade argon over molten K the surface darkens somewhat.

For moderate flow rates, welding argon, which is bubbled through a short column of NaK alloy, will do.

DSCN0826.JPG - 217kB



[Edited on 9-6-2012 by Zan Divine]




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[*] posted on 9-6-2012 at 08:42


Quote: Originally posted by MyNameIsUnnecessarilyLong  
This is the Welch 1376 style vacuum pump I'll be using: [...] Way too heavy IMO when it only pulls down to 1 x 10-4 torr.
There are two figures of merit for vacuum pumps: ultimate pressure and pumping rate. A pump of that size has a rather large pumping rate. It could be used quite successfully to back up a diffusion pump.
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[*] posted on 9-6-2012 at 08:44


Quote: Originally posted by Zan Divine  
Fish spline beads (also as fish spine beads) are sold at numerous outlets. Below is a picture of the 4.3 mm OD beads and the 0.8 mm Alumel wire I used. This is the ideal answer to the need to wrap flexible heat around a metal tube.
Another method is to adhere ceramic paper with sodium silicate directly onto the chamber being heated. After that, resistance wire can be coiled directly onto the surface. A second layer of paper/silicate holds the heating element in place.
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[*] posted on 9-6-2012 at 10:20


How will the metal cooler (I presume) be successfully coupled to the glass receptor?



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[*] posted on 9-6-2012 at 13:16


Quote: Originally posted by Zan Divine  
Verry nice! Good welding skills, too.

The metal shroud around the outside of the heating unit is beautiful but may be counter-productive. It won't reflect much heat inward but will conduct much heat outward (even if it's SS). Do yourself a favor and buy some of the bio-soluble ceramic fibre board in the low temp version. You can cut it with a knife, glue it with furnace cement and it is perfect for this application.


I wish I could claim those welds! :mad: These cones were fabricated for a local medical research institute. I'm only using it because I've had several of them laying around for years. It does look nice but if it's too problematic then I wouldn't mind replacing it.

Quote:
Don't forget the argon. Welding gas, as you know, can be up to 1% O2. Use UHP if you can. If I blow my welding grade argon over molten K the surface darkens somewhat.

For moderate flow rates, welding argon, which is bubbled through a short column of NaK alloy, will do


Do you think those argon cartridges for preserving wine are pure enough? I'd rather not buy a new tank or refill one right now. Or bubble through precious NaK :o



Quote: Originally posted by blogfast25  
How will the metal cooler (I presume) be successfully coupled to the glass receptor?


I'll be machining the taper at ~70 degrees (temp) and pass air over it during distillation to keep it from expanding or contracting beyond what it was machined at. And it'll be used with grease or sleeves of course.

If that doesn't work out, then since there really won't be high temps involved at that end, I could always put a couple o-ring grooves on the taper and have these seal it.



I would actually like for the cesium to collect directly into an ampoule so that it can be flame sealed on spot instead of transferring and flushing with argon. Hmmm... Anyone got ideas on how I could set this up?



Quote: Originally posted by watson.fawkes  
Quote: Originally posted by MyNameIsUnnecessarilyLong  
This is the Welch 1376 style vacuum pump I'll be using: [...] Way too heavy IMO when it only pulls down to 1 x 10-4 torr.
There are two figures of merit for vacuum pumps: ultimate pressure and pumping rate. A pump of that size has a rather large pumping rate. It could be used quite successfully to back up a diffusion pump.


But for such a small chamber how much does the flow rate really matter?


Speaking of diffusion pumps, I have a little Veeco diffusion pump booster like this one that's kicking around somewhere:



Except I have no idea how to install it. How much vacuum do you think this would pull when used along with my Welch pump?

[Edited on 6-10-2012 by MyNameIsUnnecessarilyLong]
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[*] posted on 10-6-2012 at 07:51



Do you think those argon cartridges for preserving wine are pure enough? I'd rather not buy a new tank or refill one right now. Or bubble through precious NaK :o

Well, probably not. But that's a guess as I'm not familiar with the product.

Here's Cs's dirty little secret, though....Cs is actually the same color as the other group 1s, bright silver. Cs forms a multiplicity of oxides. One or more of them cause the golden tint associated with Cs. So, your atmosphere doesn't need to be perfect, just good.

VERY IMPORTANT: Don't use your inerting gas in a flow-through configuration. Just apply the blanket to the vacuum of the reactor (and subsequent vessels) and keep it static.


If that doesn't work out, then since there really won't be high temps involved at that end, I could always put a couple o-ring grooves on the taper and have these seal it.

Needs good execution to seal well, but I see you've got the skills. The surface finish of the grooves should be very smooth.
Supplement with thin grease film.

I would actually like for the cesium to collect directly into an ampoule so that it can be flame sealed on spot instead of transferring and flushing with argon. Hmmm... Anyone got ideas on how I could set this up?

I can give you diagrams. I thought I had uploaded this stuff a few pages back, but maybe I'm mistaken. You'll need glassblowing skills.

If you just want one big ampoule under vacuum, simply glassblow a female ground glass joint on to the top of your ampoule. Make the constriction carefully. It needs to be sealed under a full vacuum. If Cs wets the constricted area, use gentle flame heating to evaporate it away before sealing. This is all or nothing time, if you have a blow-in, it's very unfortunate.

I prefer sealing under argon. You can just fill the above described ampoule with argon at slightly reduced pressure and it is much safer. Hard to avoid using argon in this work.


But for such a small chamber how much does the flow rate really matter?

Not at all, almost. He was just explaining why your pump was large.

Speaking of diffusion pumps, I have a little Veeco diffusion pump booster like this one that's kicking around somewhere:
IMG
Except I have no idea how to install it. How much vacuum do you think this would pull when used along with my Welch pump?

It goes between the roughing pump and the system. Vacuum depends on the medium. Oil pumps will increase your vac by an order of magnitude, roughly speaking, given your starting vacuum. Hg pumps are less effective. I can't really tell much about your unit from the picture.

Your good vacuum will allow you to redistill from pyrex. Your purity will increase and the Cs will wet glass less. One source claims that after another redistillation you'll achieve the 99.9 or 99.9+% purity necessary to make Cs non-wetting to glass.

It sounds, and is, tedious. Your distillations should proceed quite rapidly, though, which helps.

It will make your final product much more appealing. I'd do it with mine if I had the equipment.


[Edited on 10-6-2012 by Zan Divine]




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[*] posted on 10-6-2012 at 09:58


Quote: Originally posted by Zan Divine  

Do you think those argon cartridges for preserving wine are pure enough? I'd rather not buy a new tank or refill one right now. Or bubble through precious NaK :o



I doubt that too. That's low grade argon. Pull it over an oxygen getter.




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[*] posted on 10-6-2012 at 16:27


Lots of helpful info here, thanks Zan.


Quote: Originally posted by Zan Divine  

I can give you diagrams. I thought I had uploaded this stuff a few pages back, but maybe I'm mistaken. You'll need glassblowing skills.


I saw your glass contraption for purifying and ampouling Na, K. Is this the same device you're talking about?

I don't think I could fuse the two ends without constricting the neck too much. I'm using commercial pre-scored 5mL ampoules with bottle necks rather than straight tubing. The inner dia of the neck on these is only about .31"

Quote:
I prefer sealing under argon. You can just fill the above described ampoule with argon at slightly reduced pressure and it is much safer. Hard to avoid using argon in this work.


Yeah, I don't like the idea of sealing (or keeping) ampoules under heavy vacuum either. I was actually thinking that after the reaction-distillation completed, I would raise the pressure a bit first by bleeding in argon and then sealing the collection ampoule.



Quote:
Your good vacuum will allow you to redistill from pyrex. Your purity will increase and the Cs will wet glass less. One source claims that after another redistillation you'll achieve the 99.9 or 99.9+% purity necessary to make Cs non-wetting to glass.



Hmm, I thought even at that vacuum the cesium wouldn't vaporize before the critical point of the glass. Does anyone have a phase diagram of cesium?

Hell, if that's the case then I should just do the whole reaction in a glass apparatus!

[Edited on 6-11-2012 by MyNameIsUnnecessarilyLong]
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[*] posted on 10-6-2012 at 16:47


Download the file I posted on 12-15-11, Rb and Cs from the chlorides.

This is a gold mine of information. Such as when and how you can distill from glass.

You can't do the reaction in glass. Nobody even tries. Even though there is no apparent source for it, more than one prep. mentions the evolution of gas during the reaction. The vacuum during the synthesis won't be good enough to allow you to distill from glass.

Since I've recently faced a lot of the problems you'll face, I suggest you scan my posts since Dec. I hope they'll save you some effort.

Commercial ampoules have relatively thin walls. It's extremely difficult to join them to heavier glass. I'd consider making an ampoule from medium wall tubing. Remember to make it a bit long if you ever intend to open it.




[Edited on 11-6-2012 by Zan Divine]




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[*] posted on 11-6-2012 at 18:32


As I examined the reactor, something caught my eye.

You can't have a lot of thermal mass directly above the reactor. If the head is a lot cooler than the area below it, it will cause metal vapors to condense and run back down into the reactor(reflux).

The exact taper for making a metal joint can be easily looked up. I can tell you by examination of your apparatus that the metal joint won't expand enough to be a problem.




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[*] posted on 11-6-2012 at 18:56


Quote: Originally posted by Zan Divine  
As I examined the reactor, something caught my eye.

You can't have a lot of thermal mass directly above the reactor. If the head is a lot cooler than the area below it, it will cause metal vapors to condense and run back down into the reactor(reflux).



I thought about this. But I figured if I made the elbow/down-stem as close to the chamber as possible, that enough cesium would still be able to make it past the bend. The heat shield/cone might help to warm the lower flange, at least.

But would the addition/slower removal of cesium change the reduction equilibrium? I wouldn't mind if it simply required longer distillation times, but if the yield plummets...


A different reactor I was planning involved immersing the reactor and part of the condenser inside an annealing oven to give controlled and uniform heating. The condenser would stick out through a hole in the side of my oven where the thermocouple/peep hole goes. I could do the same thing to this one by throwing it in the oven, but that really defeats the purpose of the design of a stand-alone, plug-it-in cesium still...

[Edited on 6-12-2012 by MyNameIsUnnecessarilyLong]
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[*] posted on 12-6-2012 at 05:38


Quote: Originally posted by Zan Divine  

You can't have a lot of thermal mass directly above the reactor. If the head is a lot cooler than the area below it, it will cause metal vapors to condense and run back down into the reactor(reflux).



Preheat the reactor top, prior to loading, assembly and reaction, then keep a hairdrier pointing to it? 'Medium curls' should do it! ;)




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[*] posted on 12-6-2012 at 14:05


But seriously though, don't underestimate this point. Note the emphasis placed on this point on pp 962-963 by Donges in that reference.....

The equilibrium will go where it will go, the rate of take-off should have little effect on this...but...this is the ideal situation. In reality time is not your friend, you will have various processes working against you. The inevitable result of this design will be a lower yield, IMHO.

The idea of putting everything into the furnace is much better. I guarantee that it will work better.




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[*] posted on 13-6-2012 at 04:13


Do we know the BP of Cs in these conditions?

Your point is valid, Zan. Just a simple plate as reactor top would be better. Or tapering of towards the top (instead of the reverse) must be better too.




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[*] posted on 15-6-2012 at 13:26


Cs bp (760 mm) is 671 degrees C

P-T Nomograph gives a bp of 392 C @ 1 mm Hg, 336 C @ 0.1 mm Hg and 297 C @ 0.01 mm Hg.

These numbers suggest that borosilicate and a standard oil-filled lab pump are sufficient to redistill. I guess I will try this after all.



[Edited on 15-6-2012 by Zan Divine]




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[*] posted on 16-6-2012 at 04:50


Quote: Originally posted by Zan Divine  
Cs bp (760 mm) is 671 degrees C

P-T Nomograph gives a bp of 392 C @ 1 mm Hg, 336 C @ 0.1 mm Hg and 297 C @ 0.01 mm Hg.

These numbers suggest that borosilicate and a standard oil-filled lab pump are sufficient to redistill. I guess I will try this after all.


[Edited on 15-6-2012 by Zan Divine]


Thanks, Zan, very interesting. Book marked.

[Edited on 16-6-2012 by blogfast25]




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[*] posted on 16-6-2012 at 06:50


Funny thing, blogfast, is that I actually bought the extra glassware needed to redistill this way and planned to do it. As time passed, my recollection seemed to be that it would be pushing the envelope.

Now that I've actually looked at it again, thanks to your querry, It seems to be well within the borosilicate realm. Still, I'm glad the distillation pot will be buried in alumina powder inside the confines of the furnace.




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[*] posted on 22-6-2012 at 07:29


I did another of these MCl reductions last night.

It made me remember exactly how powerful the cooling effect of the metal above the reactor is on influencing distillation rate. Even with my tiny thermal mass, warming with a propane torch was needed to stop refluxing and promote distillation.

If you leave your present reactor out of the oven, you'll NEED a propane torch to do the same.




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[*] posted on 27-6-2012 at 12:06


Well, to wrap up my progress here I'll post this final picture of my recent Cs ampoules. I can now make them without any traces of oxide. Not surprisingly it a matter of learning a few little tricks, but the most important seemed to be to freeze the Cs before sealing the ampoule.

cs3 (Small).jpg - 79kB

[Edited on 27-6-2012 by Zan Divine]




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[*] posted on 27-6-2012 at 15:31


That's the first sample of silvery caesium I've ever seen. It's beautiful. :o

Do you think it'll tarnish if it were to remelt inside the ampule?

In any case, congratulations.
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[*] posted on 27-6-2012 at 16:01


Thanks. The metal remains totally shiny when melted. It looks like clean mercury but with a golden tint. It won't change since it's under argon and any adventitious oxygen has already reacted.



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[*] posted on 27-6-2012 at 16:21


Excellent.

Now if only shipping rates weren't so absurd for some of the more interesting elements...
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