Characterising neodymium magnets - Page 1
Forum section: electronics / general

author: Barry Hardhouse
author_rank: Contributor
timestamp: February 27, 2023, 02:10:57 pm
content: Hi All,Got an oddball problem and would be interested in your thoughts.We purchase neodymium magnets in an un-magnetised state. These are incorporated into an assembly and then magnatised ("charged").We have noticed that after charging, some of these magnets are failing to produce the expected flux. Everything else is identical, so we suspect that the material is substandard.I'm looking for possible ways to separate "good" magnets from "bad" prior to production (with a view obviously to getting the supplier to sort the issue out).Does anyone have any ideas how this could be done?I was hoping to measure some differences in permeability or see some hysteresis loops. However, it looks like the material has pretty much the same permeability as air - all my measurements are in the noise. Presumably you'd see the permeability jump as the magnetisation happened?Thanks!

author: mendip_discovery
author_rank: Super Contributor
timestamp: February 27, 2023, 06:54:59 pm
content: There are a few bits of kit you can buy, a gauss meter is one.A simple option might just be a fish scale and a ball bearing. Measure the force needed to remove the bearing and you are good.I found that out just googling it the other day when I was asked about a magnet for catching metal particles put of a beer mix.

author: Barry Hardhouse
author_rank: Contributor
timestamp: February 28, 2023, 01:44:54 pm
content: Thanks Mendip.Unfortunately, the Gauss meter is already reveling the problem. We can measure when the magnetised parts are weaker than expected,after magnetising, but by that point the entire assembly has been put together and must be thrown away if the magnet is no good.I'm looking for a method that might reveal if the neodymium is goodbeforeit is permanently magnetised.

author: TomKatt
author_rank: Frequent Contributor
timestamp: February 28, 2023, 02:19:15 pm
content: Absolutely no experience in this field, but my first thought is what types of QC testing does the raw material manufacturer perform and what specifications you require of the raw materials you purchase.Also, perhaps there is some kind of chemical analysis that could identify differences between known good and failed materials, such as spectrum analysis?Edit - I'm assuming you've ruled out any possible variables in your processing? Are you certain it's the raw material, or could something affect manufacturing?In any case, discussing this issue with the manufacturer / supplier would be on the top of my list - sometimes they have access to testing resources that you may not have available.

author: jmelson
author_rank: Super Contributor
timestamp: February 28, 2023, 04:10:54 pm
content: Quote from: Barry Hardhouse on February 28, 2023, 01:44:54 pmCan't you charge the magnet before assembly? Maybe you can charge, test and then "erase" before the traditional procedure.Jon

author: jmelson
author_rank: Super Contributor
timestamp: February 28, 2023, 04:15:22 pm
content: Quote from: Barry Hardhouse on February 27, 2023, 02:10:57 pmYup, you need a WAYY stronger field to see the hysteresis on that kind of magnet. You have to at least push it partway to taking a permanent field before you see the hysteresis. Maybe something like a microwave oven transformer with a couple turn secondary of heavy wire, then a couple turns around the magnet.Jon

author: duak
author_rank: Super Contributor
timestamp: March 01, 2023, 03:59:05 am
content: I take it that the magnetic circuit has been designed and assembled to apply sufficient MMF to magnetize the material? eg., sufficient cross sections, minimal air gaps and appropriate backing iron?If the MMF is applied as a pulse, is it long enough to allow any eddy current to dissipate?Is the material isotropic or anisotropic and is it oriented correctly? If you're not sure, read this:https://www.stanfordmagnets.com/magnetic-materials-isotropic-vs-anisotropic.htmlCheers,

author: Infraviolet
author_rank: Super Contributor
timestamp: March 01, 2023, 07:15:36 pm
content: Is there no way to magnetise them before they get put in the assembly? And can you be sure it is a property of the unmagnetised magnets which is responsible for some failing, and not perhaps a variability in the magnetising apparatus? Do failed magnets often occur in the same physical location within an array of magnets?

author: wraper
author_rank: Unknown
timestamp: March 01, 2023, 07:21:11 pm
content: Neodymium magnets have preferred magnetization direction, so it could be something related to that rather than material itself.

author: Berni
author_rank: Super Contributor
timestamp: March 01, 2023, 08:04:48 pm
content: Yeah the feature of permanent magnet material is that it is purposely very difficult to magnetize. That way once it is magnetized it is difficult to demagnetize.Id say the best test would be to make jig for producing small test magnets from the unknown quality material and then measuring those. Making a strong enough field for magnetizing them might involve discharging some big beefy capacitors trough a coil of wire.

author: james_s
author_rank: Super Contributor
timestamp: March 01, 2023, 08:18:24 pm
content: Won't they demagnetize if heated beyond the curie point? Does that damage them or can they still be magnetized again?

author: RoGeorge
author_rank: Super Contributor
timestamp: March 01, 2023, 08:57:29 pm
content: I have no experience with magnets, but to inspect the alloy composition, maybe buy or borrow a hand-held X-ray gun. Something like this (though no idea if that would be of any help for your application, or what model would be more suitable):https://alloytester.com/xrf-alloy-analyzers

author: wraper
author_rank: Unknown
timestamp: March 01, 2023, 09:45:39 pm
content: Quote from: RoGeorge on March 01, 2023, 08:57:29 pmThat won't work non destructively because they have nickel plating. Again, quite likely it's because of grain direction rather than material composition.https://www.kjmagnetics.com/blog.asp?p=magnetization-direction

author: thm_w
author_rank: Super Contributor
timestamp: March 01, 2023, 09:46:59 pm
content: Quote from: james_s on March 01, 2023, 08:18:24 pmThat should work: magnetize, test, induction heat to 400C, then put back into stock.Assuming the issue is the raw material and not some sort of directional effect mentioned above.https://backend.orbit.dtu.dk/ws/portalfiles/portal/215615945/berat_09115679.pdf

author: Barry Hardhouse
author_rank: Contributor
timestamp: March 03, 2023, 01:52:09 pm
content: Thanks for the input everybody.Unfortunately the magnet can't be magnetised before assembly. The strength of the magnet just makes it impossible to handle.Neither can we demagnetise after a "trial run". For reasons I don't quite understand, our demagnetising machine (pulses of alternating polarity and diminishing magnitude) won't touch them. Annealing is also off the table because, as someone said, it spoils the plating.The remark about a preferred magnetising direction is interesting though - I'll look into that.After a discussion with the engineer who's field of expertise this is, it occurred to us that this may not be a problem with the neodymium at all - it could also be a variation in the steel that also forms the magnetic circuit. We're investigating that, too.To TomKatts comment about QC: Absolutely! This is a QC issue somewhere up stream from us,and in the long term someone will have to take responsibility for having supplied substandard materials. But, as usual, we have a huge stock and we need to keep production moving or everyone in the factory might as well go home. So it's just a temporary work-around until we can prove what's wrong and get the responsible party to fix it.

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2023, 06:32:02 pm
content: Would heating still spoil the plating if done in an inert environment? Pyrex tube flushed with argon or something might do the trick.

author: ejeffrey
author_rank: Super Contributor
timestamp: March 03, 2023, 07:31:43 pm
content: I would definitely be looking at isolating the cause better: the magnet, the iron, or the magnetization procedure.Can you safely (but destructively) remove the magnets from a system? Pull one out and compare it's magnetic field to that of a known good magnet. You should be able to see if the field is weaker or in a different direction.Annealing and remagnetizing might destroy the nickel coating, but it would still be worth trying on a reject to see if it magentized properly the second time. That would indicate a problem with the magnetization protocol.Alternately: you can't degauss them and you can't heat them up to the curie point without destroying them, maybe you can do a combination. Heat them up to 150-200C and then try to degauss? I'm not at all an expert in rare earth magnets, but I would expect it becomes easier to demagnetize even well below the curie point.

author: thm_w
author_rank: Super Contributor
timestamp: March 03, 2023, 09:35:28 pm
content: Quote from: ejeffrey on March 03, 2023, 07:31:43 pmThe link I posted above used 200C, which should be fine for the plating."This temperature is considerably lower than the Curie Temperature of about 300 °C, illustrating that heating above the Curie temperature is not needed to obtain 90% demagnetization."

author: hanakp
author_rank: Regular Contributor
timestamp: March 04, 2023, 11:34:56 pm
content: If you confirm there is a probem with the magnets themselves, there is a way to quickly and non-destructively check their elemental composition, but you won't like it. It's called X-ray fluorescence spectrometry and there are even handheld "guns" that can do it. But they're quite expensive and you need to observe rather stringent ionizing radiation precautions around them. In your case, even small flaws in elemental composition may cause the problem. Thus you may need a full-sized cabinet XRF machine, which have higher sensitivity, but are even more expensive than the "guns".

author: james_s
author_rank: Super Contributor
timestamp: March 04, 2023, 11:56:41 pm
content: I saw one of those at a scrap metal dealer once. He didn't seem to be exercising any particular caution, he was just pointing it at various objects to check the composition. I think the xray dose produced by them is very low. Neat gadget, about the closest thing we have to a Tricorder in real life.

author: wraper
author_rank: Unknown
timestamp: March 05, 2023, 08:14:19 am
content: Quote from: hanakp on March 04, 2023, 11:34:56 pmAgain. They are nickel-copper-nickel plated. So you will be mostly measuring plating, not what's under it. XRF is not a magical device which measures what's deep inside of metal alloys.https://uchop.vscht.cz/files/uzel/0037768/0021~~iwhyUyhIzUstKUosyczPU0hJLSjJAAA.pdf?redirected

author: james_s
author_rank: Super Contributor
timestamp: March 05, 2023, 08:19:59 pm
content: You could strip the plating from a handful of sample magnets and test them. Or magnetize some samples without assembling into the final product and then test them and do further analysis on any that are not as strong as expected.