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Author: Subject: SmCo stir bars worth it?
monolithic
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[*] posted on 28-7-2020 at 17:50
SmCo stir bars worth it?


While attempting the NurdRage sodium synthesis my cheap PTFE stir bar (probably Alnico) suddenly died at around 170 C. Kind of unexpected because I've taken this stir bar to 200 C without issue before, but I guess everything has a useful life.

SmCo stir bars claim to be better at withstanding high temperatures but they're more expensive: $30 for a 2" x 3/8" at BelArt. Are they worth the cost and do they actually withstand elevated temperatures without demagnetizing? I'm torn between spending $30 on a stir bar or just hacking together an overhead stirring setup for $75.

[Edited on 7-29-2020 by monolithic]
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DavidJR
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[*] posted on 29-7-2020 at 03:53


No idea, but i've definitely found that cheap stir bars from China (eBay) are more prone to demagnetisation than ones obtained from big-name lab suppliers. I assume they must use a different kind of magnet material but I do not know the details. I've never spent that much on a single stir bar though.

[Edited on 29-7-2020 by DavidJR]
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Dr.Bob
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[*] posted on 29-7-2020 at 16:55


I don't think I have ever tried one in all my years in the lab, but I would be curious to know if they actually work well, so maybe you could test one as a public service. I am curious if they stir better than a similar sized normal stirbar, as well as their durability and heat resistance. Is the 2 x 3/8 the hexagonal one or a oval or something in between? I have some large ovals, but they are more like 2 x 5/8", which will not fit into a 24/40 joint. If you get one, I would be willing to give you a few Belart brand normal ones to compare with it, if that helps.
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monolithic
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[*] posted on 29-7-2020 at 19:02


Quote: Originally posted by Dr.Bob  
I don't think I have ever tried one in all my years in the lab, but I would be curious to know if they actually work well, so maybe you could test one as a public service. I am curious if they stir better than a similar sized normal stirbar, as well as their durability and heat resistance. Is the 2 x 3/8 the hexagonal one or a oval or something in between? I have some large ovals, but they are more like 2 x 5/8", which will not fit into a 24/40 joint. If you get one, I would be willing to give you a few Belart brand normal ones to compare with it, if that helps.


It's octagonal, you can see it here https://www.amazon.com/Bel-Art-Spinbar-Magnetic-Stirring-F37...

The reaction calls for 30-40 hours of stirring at 200 C in an atmosphere of NaOH and molten sodium, which tends to noticeably degrade PTFE. I just ordered some new crappy PTFE stir bars. I'm going to try it one more time with crappy stir bars before I resort to potentially destroying a $25-30 stir bar. :o
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Sulaiman
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[*] posted on 29-7-2020 at 21:46


I too would like to know if ptfe/samarium cobalt stir bars are better than the common ptfe/AlNiCo types.

Re: typical cheap Chinese overhead stirrer parts,

Blades/paddles are pure ptfe so may be a little soft at higher temperatures.

Shaft is steel with a ptfe sleeve, corrosion at the bottom of the Shaft may occur?
(I've not used mine much yet so I don't know)

The rotating seal is ptfe in a greased O-ring,
I know it is OK at atmospheric pressure and 20kPa (150mm Hg) pressure,
but I suspect not so good for sealing lower pressures.

So for your purposes, I guess that a nice SmCo stir bar may be the best option..




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[*] posted on 30-7-2020 at 02:36


Alnico is more easily demagnetised than SmCo, but is therefore also more easily remagnetised. A coil of wire (single layer of fat turns) and a big chunky capacitor should do it. Remag your stuffed stirbar instead of replacing it.

Both SmCo and Alnico have similar operational temperature ranges.




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unionised
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[*] posted on 30-7-2020 at 04:25


Quote: Originally posted by Twospoons  
Alnico is more easily demagnetised than SmCo, but is therefore also more easily remagnetised. A coil of wire (single layer of fat turns) and a big chunky capacitor should do it. Remag your stuffed stirbar instead of replacing it.

Both SmCo and Alnico have similar operational temperature ranges.

Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximal working temperature is normally limited to around 538 °C (1,000 °F).
from
https://en.wikipedia.org/wiki/Alnico
vs

Good temperature stability (maximum use temperatures between 250 °C (523 K) and 550 °C (823 K); Curie temperatures from 700 °C (973 K) to 800 °C (1,070 K)

from
https://en.wikipedia.org/wiki/Samarium%E2%80%93cobalt_magnet

I think that difference in working temperatures may be significant in some cases, but the PTFE will be starting to soften.

You used to be able to buy (and I guess you could still make ) glass covered stir bars. (if you make one, remember to ensure that the magnet is a lose fit in the tube or thermal expansion might catch up with you.
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monolithic
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[*] posted on 30-7-2020 at 05:46


Quote: Originally posted by unionised  

You used to be able to buy (and I guess you could still make ) glass covered stir bars. (if you make one, remember to ensure that the magnet is a lose fit in the tube or thermal expansion might catch up with you.


I considered this but looking at reviews on Amazon it seems they break easily and pre-made ones are hilariously overpriced: https://www.amazon.com/Glass-Encased-magnetic-Stir-Bars/dp/B... . Borosilicate on borosilicate contact for 30-40 hours in a caustic environment at 200 C also sounds risky. The more I think about it, the more practical overhead stirring seems. No risk of magnetic decoupling and if the paddle softens and gets destroyed just throw it out and buy another one for $10 off eBay.

[Edited on 7-30-2020 by monolithic]
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[*] posted on 30-7-2020 at 06:29


d.c. / permanent magnet motors make excellent stirrer motors as they can give near full torque down to zero speed,
with the fire/explosion risk of sparking commutator brushes.
I got an excellent motor from a damaged hand mixer, my YT video;
https://www.youtube.com/watch?v=ziRxiFSaZsA&t=2s




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monolithic
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[*] posted on 30-7-2020 at 07:04


Quote: Originally posted by Sulaiman  
d.c. / permanent magnet motors make excellent stirrer motors as they can give near full torque down to zero speed,
with the fire/explosion risk of sparking commutator brushes.
I got an excellent motor from a damaged hand mixer, my YT video;
https://www.youtube.com/watch?v=ziRxiFSaZsA&t=2s

Nice, simple and effective. I don't have a variable DC power supply -- wonder how well a cheap PWM controller would work. :)
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[*] posted on 30-7-2020 at 07:06


You'll end up considering buying or making an overhead stirrer at some point anyway.

I know I am going to make one. I already bought some teflon paddles from Deschem.

I dont know what reaction you have planned that would need an overhead stirrer because of viscosity but that's only one reason.
The price difference between heating mantles with or without stirring is mad while adding an overhead stirrer well... suddenly RBF stirring isnt as costly anymore.




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[*] posted on 30-7-2020 at 08:27


Me three. Got a paddle and few different sizes of PTFE joint bearings. They should also allow vacuum use.

Main reason is that any device with stirring is not capable of heating enough. A powerful and reliable enough hotplate will cost up from 400 euros and heating mantles are very flask size sensitive.
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[*] posted on 30-7-2020 at 16:39


Quote: Originally posted by monolithic  
I don't have a variable DC power supply -- wonder how well a cheap PWM controller would work. :)


They should be perfect for the job.




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[*] posted on 30-7-2020 at 16:47


Quote: Originally posted by Sulaiman  
d.c. / permanent magnet motors make excellent stirrer motors as they can give near full torque down to zero speed,
with the fire/explosion risk of sparking commutator brushes.
I got an excellent motor from a damaged hand mixer, my YT video;
https://www.youtube.com/watch?v=ziRxiFSaZsA&t=2s


Excellent work, well done.

The prices on these things at lab suppliers is ridiculous.

Have you tested it to see how it performs with continuous use for several hours?

[Edited on 31-7-2020 by Eddie Current]
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[*] posted on 31-7-2020 at 01:08


no, the longest that I ran it was about 4 hours,
the motor got warm, but never hot.
I guess that the O-ring seal will wear out long before the motor.




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[*] posted on 31-7-2020 at 06:45


glass tubing can be used to make an overhead stirrer.
Because it is relatively thin, it isn't subject to a lot of thermal stress when being use as a stirrer.
This is what everyone use before PTFE covered magnets.

As for the type of magnet.
Alnico has a higher curie temperature than SmCo
But many stir bars are neodimium which has a curie temperature around 320C maximum and starts losing magnetism before that.
Glass covered Alnico are readily available and work but are weaker than standard stir bars.
They are good up to the softening point of borosilicate glass.
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[*] posted on 9-8-2020 at 21:08


Seems to me, a Neodymium Magnet loses its mojo, at under the BP of water. Powerful magnetic coupling

Samarium Cobalt, has much better heat resistance. Pretty good magnetic coupling.

Alnico is better still, in terms of heat resistance. Worst of the three in terms magnetism.

For what you are doing, you need maximum heat resistance. Magnetic coupling is of secondary importance.

You don't need to entrain a gas or etc., so high RPMs and powerful coupling are not required.

Just stir it. Your stir bar gave up the ghost? Get another cheapo. This reaction eats stir-bars.

It's the cost of doing business. Maybe, a magnet inside of a stainless steel tube will hold up. Stainless screws in the ends. Available at your local hardware store. Couple of bucks. Threadng the inside of a stainless steel tube though; not easy.

[Edited on 10-8-2020 by zed]
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monolithic
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[*] posted on 20-8-2020 at 11:14


Quote: Originally posted by Sulaiman  
Quote: Originally posted by monolithic  
I don't have a variable DC power supply -- wonder how well a cheap PWM controller would work. :)


They should be perfect for the job.


Ended up ordering some parts from China and tested it on a bucket of water, about 2 gallons. Has no trouble forming a very strong vortex even at low power. Kind of violent at full RPM but it's just a matter of keeping the knob turned down. Thanks for the suggestion!

Parts list if anyone cares to build one:
https://www.banggood.com/775-Motor-DC-12V-36V-3500-9000RPM-M...
https://www.banggood.com/1_5-10mm-Electric-Drill-Chuck-with-...
https://www.amazon.com/gp/product/B00DVGGWC0/ref=ppx_yo_dt_b...

Plus an 12V power supply, some wire, and a PTFE stirrer I bought from China a few years ago. Motor draws <1 amp under load so even a wall wart would work. Pic of vortex (sorry for bathroom shot): https://i.imgur.com/Il2Yhn6.jpg

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Sulaiman
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[*] posted on 21-8-2020 at 06:02
Nice


Just remember to tighten the chuck well,
a few hours ov vibration could loosen it.




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