Ferrite resistivity

Probably the resistivity is quoted mainly because it affects inductor Q. Ferrites are, alas, composite materials; the magnetic grains (of hopefully uniform size, big enough to magnetize) are in a matrix of fired ceramic like kaolin clay. So, electric conduction depends on percolation through the ceramic matrix, or through contact between grains, or in the boundary layer where the kaolin is in contact with the hematite/magnetite. =20

It is dubious that resistivity repeats from batch to batch, or from one con= tact point to another. A bit of internal fracture might not change the magnetic properties much, but would kill a resistivity measurement. I've done impedance measurement on ferrites that had LOTS of internal movement, at some frequencies the internal fractures rang like a lossy bell.

For a 2-terminal measurement, the contact footprint, down to the microscopic level, may matter.

What's soft, compliant, and has high conductivity?

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Zebra strip?

Farnell don't seem to stock it any more, so you may find it difficult get hold of.

The Kelvin connection scheme is always attractive.

-- Bill Sloman, Nijmegen

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I did stick the extreme pieces of 73 on an SRS RCL meter. (1.2k and

So if most of the resistance is because of some random path... I still should be able to see any phase change in the resistivity.

George H.

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Sure, the resistance changed as I cranked on the screw. But only by maybe a factor of 2 or 3. I could tighten things up to within ~20% of the cracking point.... after I'd cracked a few beads.

Oh yeah this was very crude... the spinning of the screw was the worst part.

I tried using some 'gummy' copper tape as a 'gasket', but it spun off into a crumple ball.

Hmmm.. those beads aint so big, making two more (ring?) terminals would be real work.

George H.

ed text -

Although this is a useful method to differentiate the two materials, it's not going to give meaningful bulk resistivity information.

The higher resistivity material is sintered in a fairly crystaline structure, with insulator/semiconductor type relationships between domain groups. You can't count on it functioning as an insulator in-circuit, because this characteristic is uncontrolled - but you sure can count on the low resistivity stuff causing shorts, if misapplied in a higher voltage circuit.

Some materials are not formed as individual beads, but are machined from tubular structures, or finished into their final dimensions by grinding, The machined parts provide a more reliable contact to the bulk material, but their function is also marginally affected due to the reduced bulk impedance at the machined surface.

It used to be that low resistivity, low frequency parts also had noticably lower currie temperatures. This restricts uses of parts in places where significant power loss is expected - they become loss-self-regulating, much like an NTC resistor - which is no good if a loss must be absorbed and the site of application isn't heatsunk in some way.

RL

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Thanks, If this works I can try looking for better samples. The ferrite beads are probably the 'low end' of the production run.

A low Curie temperature would be attactive. (easier to reach) I like this melamine foam which is good to?? (at least 140 C)

George H.

Cylindrical, flat contacts would be best. Lead or solder foil might be a nice, soft, sorta compressable contact. Or mercury!

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Hmm, I think the first thing I need to get rid of is the twisting motion of the screw. I don't quite see how to do that. (And keep the large adjustment range.)

Now here's a question for your four point measurement suggestion.

If I've got some random walk conduction through the sample... (to explain the large resistance) Then when I put on my two voltage probes, do they make random walks through the sample too? In which case the position of the volatge probes becomes only an approximate measure of the distance along the conduction channel.

Hey, can I do some sort of AC measurement that probes shorter range conduction? Or is that at a very high frequency?

George H.

ed text -

My ferrite knowledge is about 40 years old, from the Philips plant where I worked as ferrite lab engineeer, so there are probably newer compositions on the market.

Ferrites are solid solutions of either manganese or nickel and zinc and iron oxides, sintered to form micro crystals around 1 micron size. Each crystal is intended to be one magnetic domain, for lowest losses. There are minor but well controlled additions of silica and/or calcium oxides which act as glassy insulators between the ferrite grains. In the manganese-zince ferrites there is also a critical amount of FeO in the inter-grain spaces, whereas the grains themselves are MnO2-ZnO-Fe2O3. ( Magnetite, Fe3O4 is really a solid solution of FeO in Fe2O3, ie its a 'ferrite' without the manganese, nickel or zinc components ).

Basically there are two formulations:

Manganese zinc ferrites For low frequency, say 1 KHz to 200 KHz. High permeability, around 2000 and up, low resistivity, around 100-1 kohm-cm or so. Curie temp around 150-200 degC, depending on formulation ( manganese-zinc ratio ).

Nickel zinc ferrites For high frequency, say 500 KHz to 50 MHz. Permeability around 50-500, high resistivity, around 1e9 ohm-cm or higher. Curie temp 200-500 degC, again depending on formulation ( nickel-zinc ratio ).

Resistivity is not well controlled, and only has a small part in ferrite losses. It also has very high negative, and very uncontrolled, temp coeff.

Ferrites are optimised for work around -20 to about +100 deg C. Below that, the permeability falls rapidly, and there may well be phase changes, as you already discovered.

Yes, we had some offline discussion with George about the phenomenon. The microscopic mechanism behing Verwey transition is quite intriguiging, and its existence does not seem to be widely known

- at least I had never heard about it before (well, if its covered in Kittel that is just my ignorance, we followed Aschroft-Mermin...). The ferrites I have tested in LHe have all been the standard MnZn or NiZn spinel types (it seems hard to find other types, like garnets, sold in small quantities) and it sounds likely that it was exactly the Verwey transition which killed them - assuming the transition also happens in mixed spinels, the papers I've found only describe pure magnetite.

Anyone knows an easy small-quantity source for microwave ferrites, by the way?

Regards, Mikko

I read a paper where someone 'measured' the precession rate of the 'available magnetic molecules and it was in the range of 1 to 2 GHz, which the author used to explain why there are no microwave ferrites.

In my experience, 1 GHz was about the upper limit, air core above that was far more effective, and for EMI graphite..

Hi Robert, thanks for your insight. Do you happen to remember a reference to the paper?

To me it looks that Mini-Circuits transformers do use magnetic cores, see e.g. http://217.34.103.131/pdfs/TC1.5-1X+.pdf http://217.34.103.131/pdfs/TCM2-33X+.pdf http://217.34.103.131/pdfs/TC4-14+.pdf

There are also all sorts of tuneable microvawe thingies which make use of YIG garnets.

Then there are circulators available for a wide variety of frequencies, those must utilize ferrite cores, no?

Regards, Mikko

MCL mostly makes transmission line transformers, so the cores are there to extend the low-frequency response. TLTs are also pretty well completely insensitive to core loss, since their main operating mode produces zero core magnetization.

Cheers

Phil Hobbs

Minicircuits makes both TLT:s and ordinary ones. Eg. http://217.34.103.131/pdfs/TC1.5-1X+.pdf is an ordinary transformer (judging by the 'Config D' in the data sheet), and the in the photograph it looks to me like having a magnetic core.

An example of their TLT's is http://217.34.103.131/pdfs/TC4-25+.pdf which is clearly indicated as 'Config. H' in the datasheet.

There are a large variety of companies like

Regards, Mikko

I always thought this was very cool,

but then I'm easily amused. Slipping some ferrites over the coax, anywhere, extends the LF response.

PSPL sells this in a box for big bucks.

Adrian - great to hear something from the horses mouth for a change. Copied to my magnetics article index, with your permission, I hope.

RL

Interesting point. 1.5 isn't an available impedance ratio, so I expect it's actually 5:4 or 6:5 turns. You could do that as a mostly-bifilar winding with one extra turn--maybe that green wire in the foreground. Not as good as a real TLT, but not as bad as a split-winding toroid.

The place I mostly buy cores is Amidon, but they don't go up that high.

Cheers

Phil Hobbs

So why not undercut them? Some microwave things would be pretty cool if you could make them work well in the time domain, e.g. splitters, directional couplers, isolators, and so on. I wonder what the BW limit for practical 90 degree splitters really is?

Cheers

Phil Hobbs