=materials =physics
Neodymium magnets are the main type used in modern motors. Why are they good? Are there any good alternatives?
review of ferromagnetism
Magnetic fields contain energy. In an inductor, that energy comes from an
increase in voltage when current is first applied. When a magnetic core is
added to an inductor and a stronger field is produced from the same added
energy, that extra energy has to come from somewhere.
The energy that
ferromagnetic cores add to magnetic fields comes from their crystal
structure fitting together better in a magnetic field. This implies that
ferromagnetic cores should spontaneously magnetize to some extent, and they
actually do; it's just that the spontaneously generated magnetic fields are
curled into microscopic 3d loops. The microscopic internal field strength is
approximately the saturation field of a ferromagnetic material, which is
often greater than the field generated by a Nd magnet. Applying an external
magnetic field causes those microscopic magnetic loops to partly unroll.
The actual field is generated by unpaired electrons of atoms; individual
electrons are very magnetic. But ferromagnetism isn't a property of atoms,
it's a property of crystals; without particular crystal structures that
favor magnetic fields, those unpaired electron spins of iron atoms would
just cancel out. For example, stainless steels contain a lot of iron, but
most aren't ferromagnetic.
Atoms of crystals fitting together better
in a magnetic field implies that iron cores slightly change shape when a
magnetic field is applied.
This effect is
responsible for the humming noise transformers make, and has been used for
eg sonar.
common misconceptions
Fucking
magnets, how do they work?
And I don't wanna talk to a scientist.
Y'all motherfuckers lying, and getting me pissed.
— Insane Clown Posse
The
Insane Clown Posse is sort of right there: a lot of explanations of magnets
given to people by teachers and media scientist-figures have been partly
wrong.
Magnetic flux was originally thought to be a flow of something
like electric current, with ferromagnetic materials having lower resistance
for that flow than air. It's even still taught that way sometimes. But no,
it's a complex emergent phenomenon.
I remember being taught that
"iron is magnetic because it has an unpaired electron". But again,
ferromagnetism is a property of crystal structures, not atoms or elements.
A lot of people think the magnetism of neodymium magnets comes from
the neodymium, but the actual magnetism comes from the iron in them.
The title of the quoted song is "Miracles". The physical constants that
allow for the complex emergent phenomenon of ferromagnetism are the same
physical constants that allow for the complex emergent phenomenon of life;
most values of them wouldn't do either. The universe having values allowing
for those is indeed a miracle that nobody really knows the reason for;
thanks for reminding us of that, ICP.
neodymium magnets
In
permanent magnets, the crystal structure is such that the magnetic fields of
crystals can't rotate around to form closed loops very well.
Neodymium magnets
(Nd2Fe14B, "Nd magnets") are the strongest permanent magnets currently
available. Looking at
the crystal structure we can see rings of iron atoms with Nd in the
middle and some boron at the 3-way vertices. When a magnetic field is
applied through that (tetragonal) pattern, the atoms fit together better.
You can see how the magnetic field would be unable to smoothly rotate
through directions.
Strong Nd magnets are made by cooling inside a
strong magnetic field, so that the crystal structures are aligned in one
direction.
alternatives
An obvious
idea is using the same structure but replacing the neodymium with a
different element. That's obvious enough that people tried everything, and
neodymium is the best option.
Or, you can go for a different crystal
structure. You need something that's different in 1 direction than the other
directions. Considering the common crystal structures and the goal here,
tetragonal and hexagonal structures are the obvious options.
One of
the best permanent magnets besides Nd magnets is
SmCo,
which is hexagonal. SmCo magnets are weaker at room temperature, but
stronger at higher temps, so they're better for a few applications. They
require a lot of samarium so they're even more expensive than Nd magnets.
FeCo isn't a good permanent magnet, but has a really high saturation
field. Perhaps FeCo with some sort of microscopic structure that prevents
field rotation could be a good permanent magnet? Yes, actually; the
magnetism of Alnico magnets
comes from needle-shaped FeCo nanoparticles in a non-magnetic
nickel-aluminum matrix. But of course, Nd magnets are much stronger.
If FeCo in a non-magnetic matrix is a decent magnet, then logically, a
magnetic matrix would be better. So some people have worked on
FeCo / Nd magnet nanocomposites, and the results seem similar to Nd
magnets, with less Nd needed. But manufacturing such structures is currently
impractical.
There's been some research interest in iron nitride
magnets recently. Fe16N2 is quite magnetic, noteable for its high saturation
flux. But it's also metastable, with the desired structure decomposing
around 212 C. To compact it into fairly-dense magnets at 200 C, you need
extremely high pressures - impractical pressures, like 10,000+ atmospheres.
And the result is still worse than Nd magnets. If the goal is to reduce
costs, that doesn't seem very good, but the lab-scale results have been good
enough to get funding.
So, is something new going to replace Nd
magnets? I'm not optimistic about that at this time. If they do get
replaced, I think it'd involve a new manufacturing technique producing some
good microstructure, rather than discovery of some magic ratio of elements
that naturally produces better results with an existing process. In the
meantime, what I expect to happen is more mining of neodymium for magnets
for motors.
alternate alternatives
If
developing good permanent magnets that don't need Nd isn't feasible, another
option is finding ways to avoid using permanent magnets.
High-performance electric motors these days are all using permanent magnets,
but switched reluctance motors aren't bad. There are also hybrid designs
using some permanent magnets but less than current motors. Research into
electric motor configurations is still ongoing in 2024, believe it or not.
Anyway, switched reluctance motors aren't currently quite as good, but they
work well enough. (On the other hand, induction motors are now obsolete, and
Tesla Motors was dumb for using them.)
Superconductors are good at
producing persistent magnetic fields, so they can be used instead of
permanent magnets. But of course, they're expensive, and they have to be
kept cold, and they have some losses when their current changes. See also my
previous post on flux pumping. Maybe that
will end up being a viable option for very large electric motors. Or maybe
someone will find a better way to make YBCO wire that research has missed so
far, but that seems less likely.