Permeability change of Ferrites

Thermal cycling has more to do with disaccommodation but that isn't really a long term effect (it is repeatable).

Paragraph 5.1 on page 5 explains the long term drooping a bit more:

If you need good long-term stability I guess you'll have to age the cores, to get them out of the initial time-logarithmic decay. Maybe place them next to the bottles of Chateau Lafite :-)

With iron powder I believe that much of the aging effect depends on what kind of binder they use, whether it's organic and so on. A good manufacturer should be able to furnish test data for their materials.

I sometimes torch my ferrites to unglue them. And I do wonder if that fk's them up. I'm guessing I bring them up to 250C to get the CA glue to release.

It's not easy to imagine this being a problem. Sintering occurs at something like 1200C or more.

Jon

Thanks. That's comforting. :)

On a sunny day (Fri, 04 Nov 2011 09:50:41 -0700) it happened Joerg wrote in :

Interesting paper, thanks.

Jon Kirwan a écrit :

No mechanical damage for sure but ferrites are ferromagnetic material and have a Curie temperature above which they change their properties and structure.

I don't have experience about this but my guess is that, as well as with the ferroelectric ceramic caps, driving them above their Curie point will reset the disaccomodation process, as it does for the high K ceramics.

"Klaus Kragelund"

The tempco is very small and no signs exist of them drifting with age.

.... Phil

Getting back to the original question, the disaccomodation is proportional to log time, so after the first 24 hours or so since the last drive into saturation followed by demagnetisation, the effect gets rapidly too small to measure. Temp coefficient of the permeability masks the disaccomodation by far.

Ferrites are used in precison inductors, but the cores involved are invariably gapped, and the gap is always big enough that it dominates the magnetic path length. The permeability of air is little different from that of a vacuum, and the permeability of the part of the magnetic path running through the ferrite has a very limited effect on the total inductance.

-- Bill Sloman, Nijmegen

Alas, 'ferrites' are composite ceramic materials. Like concrete, made of cement and sand and aggregate, ferrites have binders and magnetic-material aggregates, and they're a LARGE variety of materials all under the same name.

What ferrites all have in common, is that they're inert after firing, so (unlike steel) won't oxidize. Aging is a big plus for these materials.

They have this rather odd figure of merit- "relative temperature coefficient", which is the temperature coefficient of the permeability divided by the permeability. They get really low numbers like 2ppm/K that way. I suppose it allows them to quote a single number (or give a single curve) for gapped cores.

So if your air gap decreases the effective permeability to, say, 50, you'll have a tempco of ~100ppm/K.

Best regards, Spehro Pefhany

"Spehro Pefhany"

... Phil

Is an air gap viable?

"Nemo"

Otherwise it becomes a transformer primary.

... Phil

You have to tell that to all those ferrite toroids :-)