lysander
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High-performance stirring magnets
Where can I get high-strength (MGOe) stirring magnets? I haven't found any stir bars that specify their magnetic strength, but they don't seem to be
in the same league as the N42+ uncoated magnets commonly seen in other applications.
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Deathunter88
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Quote: Originally posted by lysander  | | Where can I get high-strength (MGOe) stirring magnets? I haven't found any stir bars that specify their magnetic strength, but they don't seem to be
in the same league as the N42+ uncoated magnets commonly seen in other applications. |
That is because most Neodymium Iron Boron magnets are not very resistant to heat. Most start to lose magnetism at 80˚C, while the very expensive high
temperature ones can only go to about 200˚C
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lysander
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100C would be good enough for some of my applications, and 200C would cover most. So am I just such a niche that there's no market in high-magnetic
low-temperature stir bars?
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zed
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For lower temperatures. Neodymium magnets are spiffy, and cheap too. Retail, they are generally plated with a thin coat of Nickel. Not nearly thick
enough for what you plan to do. Might have to coat your own.
You could attempt to plate them more thickly, with something very corrosion resistant, or you could devise some other coating. Like, stick one
inside some neoprene tubing and plug the ends, or place one inside a thin walled stainless steel tube, and crimp.
Seems to me, I have seen Samarium Cobalt stirbars, advertised. But, they were pretty spendy.
[Edited on 25-9-2015 by zed]
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Justin Blaise
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I can attest to the fact that Co-Sm magnets can stay magnetized at high temperatures. I have one that spent several minutes under the flame of a
propane torch without losing magnetization. If you could take a Co-Sm magnet and seal it in a small ampoule, you would have a heat-resistant, albeit
brittle, stir bar.
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Upsilon
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Except that it would float in water; not really that helpful  P
You would need to fill in the empty space with sand or something. Filling it with water might not be a good idea since it may deteriorate the metal
over time.
[Edited on 26-9-2015 by Upsilon]
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careysub
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It does appear that samarium cobalt is the super magnet material of choice for this, given its higher operating temperature.
Looking around for SC magnets I see a lot of little magnets, though you can easily stack magnets to create whatever length of stack you want. However
it occurs to me that most of the torque on a stir-bar is probably generated at its ends. You could put little SC magnets attached to a piece of heavy
iron nail perhaps in a glass tube and seal it (iron is better ballast than sand anyway).
Are there good chemical resistant polymers that can be heat sealed above, say, 250 C? Teflon is one (MP 330 C), and you can by Teflon tubes. Seems
like you should be able to use heat and pressure to close a PTFE tube, they do form it in molds after all. If it did not consolidate, fuse together,
you wouldn't get a solid piece.
If Teflon fails the test (maybe sealing proves difficult?) what other candidates are there?
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macckone
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PFA or FEP is better for sealing than PTFE but PTFE is more resistant.
Glass coated stirring bars are also an option.
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zed
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You can stack magnets. You do suffer a small loss of total magnetic attraction when you do.
2x1inch<1x2inch. More units=more loss.
Equipting your stir plate with stronger magnets is also an option.
Drafting sets, used to contain a small metal capsule, for storing "graphites". About the right size to stick your magnet inside.....if you could find
one in stainless steel.
Ah, I took a tour of the internet. The world has changed since my last quest for Samarium Cobalt stir bars. They are cheaper, and more available.
https://www.google.com/search?q=samarium+cobalt+stir+bars&am...
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p205060...
[Edited on 26-9-2015 by zed]
[Edited on 26-9-2015 by zed]
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