Electronics > Projects, Designs, and Technical Stuff
Annealed cast iron as magnet pole pieces (for low-field NMR)
ChristofferB:
Hi!
I'm still working on a homogenous-ish magnet to do NMR and EPR spectroscopic experiments with, and have settled on some 10cm diameter ferrite puck magnets as my magnets (found a paper doing just that, it's nice having a reference to compare your design to).
I'm looking into how to make the puck-shaped pole pieces for it, to homogenize the field, and of course, discs of soft iron would be best, but going to specialized magnet alloy companies is slightly (=completely extremely) over budget for this project. I've found an ebay seller that sells 10 cm cast iron rod in slices, and was wondering if these would be a suitable(ish) substitute if heated to anneal them.
What do you think? Would I be better off without pole pieces entirely, or what?
I've added a schematic of the planned magnet layout, not that it's really needed but posts with visuals are just nicer aren't they?
Thanks!
--Chris
Thanks in advance!
RandallMcRee:
What are the dimensions? (10cm diameter? what about thickness?)
McMaster-Carr sells iron discs that might work.
I have lots of magnets left over from years of experiments with planar-magnetic speakers if you need magnets...just ask. No iron, though. I used some kind of steel (forget exactly what now, but cheap) which was listed in the femm database.
Do you know about FEMM?
ChristofferB:
I didn't know about FEMM but I do now! Thanks a bunch!
The magnets I've settled on are 100 mm in diameter, 15 mm thick, and I want my pole pieces to be the same.
I'm in Europe, so McMaster-Carr is pretty much out of the question, unfortunately.
I was looking at this auction: eBay auction: #https://www.ebay.com/itm/100mm-approx-4-dia-meehanite-cast-iron-round-bar-1-12-lengths/263704103005
1 inch pole pieces would be fine, too, and I'll find a local machine shop and ask if they could face them on the lathe to get flat parallel surfaces.
I'm not sure annealed cast iron has high enough permeability to work in this application, though.
thanks!
T3sl4co1l:
Annealed mild steel should be just as good, and may be easier to get?
TLDR: I forget the exact magnetic properties (if such a term isn't a contradiction to begin with :) ) of common iron products... but I also suspect that there's more variation between samples, and their condition, than might be expected. I'd prefer the simple solution (mild over cast), which is probably just as easy to obtain?
Soft gray iron should be more-or-less pure ferrite* in the metallic phase, with graphite flakes throughout (which of course don't have any magnetic effect). Avoid anything of higher strength, including ductile iron (the softest grades of which are probably fine still, but the stronger ones not so much).
*Metallurgical "ferrite", alpha iron. Soft, mechanically and magnetically. Stronger alloys, treatments even, will contain more pearlite, which is magnetically hard.
Beware that annealing cast iron is... kind of complicated. It's full of carbon, so can accidentally be hardened very easily.
Mild steel is very easy to anneal, just heat to orange-hot, then cool slowly. The slowness doesn't even really matter (as long as the carbon and alloy contents are low) -- it's hard to cause much change even water-quenching it.
Mind that cheap steels are not created equal. "A36", "rebar" and so on, are -- like many semiconductors, actually -- defined not by composition or structure, but by overall properties. As long as they meet the requirements on strength and flexibility and whatever, they're fit for that purpose; but that says nothing about their magnetic or metallurgical behavior. Preferably, stick with known 1020 mild steel (or less; the "20" means 0.20% carbon). Or 12L14 is probably fine too, and better to machine.
Silicon steel and nickel steel (possibly austenitic, I forget?) of course are better, but if you don't have machining or purchasing capability for these, well then that's that. :)
Tim
Circlotron:
In your diagram shouldn't you have another piece of ferrous material going from one outer magnet face to the other outer magnet face?
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