Magnets have been at the heart of
the furore surrounding the rare earth industry. The present and
future use of rare earth permanent magnets in key defence and
green technologies, from missiles to electric vehicles to wind
turbine generators’ have been a primary reason for the
attention that this group of elements now commands at
government level.
This is not to mention the
important role that they play in everyday items such as
computer hard disk drives, loudspeakers and other household
devices.
The most widely used magnet,
neodymium-iron-boron (Nd-Fe-B), has not been changed since its
invention in the 1980s such is its strength and performance at
high temperatures, together with samarium-cobalt (Sm-Co).
While these elements have hit the
headlines, it is actually the markets for these magnets which
is why the world is racing for rare earths.
Maintenance work on a wind turbine motor which
relies on the unrivalled strength of rare earth
magnets to function. Siemens
Why are rare earth-based permanent magnets so
irreplaceable?
Gareth Hatch: Rare
earth elements have magnetic, electronic and optical properties
not found in any other element groups in the Periodic Table.
This is primarily a result of the unique electronic structures
found in these elements.
By combining certain rare earth
elements with transition metals that exhibit magnetism, such as
iron or cobalt, we can effectively “channel” the
magnetic output of these elements, at the microscopic level,
into a single “North-South” axis, leading to very
strong magnets.
If we were to create same-sized
samples of the types of magnet materials currently available,
samples of Nd-Fe-B magnets would be able to put out the
strongest magnetic field.
We use a figure of merit to compare
magnet materials called the maximum energy product, or BHmax.
Essentially this is the maximum amount of energy that can be
stored by the magnet material, for a given volume, and is
measured in thousands of joules per cubic meter (
kJm-3) or in millions of gauss oersteds (MGOe).
Commercially-available grades of
Nd-Fe-B have the highest maximum energy products, around
52-54m. gauss oersteds (MGOe), compared to Sm-Co, which has up
to 32 MGOe, Alnico, 5-7 MGOe] and ferrite, to a maximum of 4
MGOe].
And for samarium magnets?
GH: Similar effects at the microstructural level are
observed for Sm and its interaction with Co (and other
elements), although the output is not quite as dramatic, or as
powerful.
However, Sm-Co magnets are able to
operate at much higher temperatures than Nd-Fe-B magnets before
their useful magnetic output disappears, though in recent years
the addition of dysprosium (Dy), a heavy rare earth element, to
Nd-Fe-B magnets, has helped to increase the temperature
resistance of those materials too.
It’s really only rare earth
elements like Nd and Sm (and some others) that can produce this
“channelling” effect.
Are there any other special properties?
GH: As a result of the anisotropy, Sm and Nd-based
magnets have very good resistance to being demagnetized by the
presence of other magnetic fields. This allows us to build
special assemblies with groups of magnet components that are
arranged in certain ways so that the magnetic output is
concentrated, leading to higher magnetic outputs than could be
possible with a single magnet.
One thing to bear in mind with
Nd-based magnets, in particular, is corrosion resistance.
Because of the metallurgical structure of such materials, they
are much more susceptible to corrosion than Sm-Co magnets, and
thus Nd-Fe-B magnets are usually coated or plated before
use.
Is volume of material a factor?
GH: Yes. Another way to look at the differences, tied
into the energy product value mentioned earlier, is that if you
want a certain magnetic output, then using Nd-Fe-B magnets
means you need less volume of material to do it.
In other words, you can use smaller
magnets made from Nd-Fe-B, compared to Sm-Co or other
materials, to get the same job done. So anywhere where weight
is at a premium, Nd-Fe-B would be the first choice magnet.
However, Sm-Co magnets are used in
applications where a strong output is still required, but where
the expected temperatures exceed those that Nd-Fe-B could
handle. Examples might include the magnets used in missile
sub-systems, or in the oil and gas drilling industry, where
magnetics are used “downhole” as part of the
surveying process. Note though that Nd-Fe-B magnets can still
be used in a number of weapons systems.
The strength of rare earths
Neodymium based magnets are nearly double the strength of
samarium
magnets and can be ten times stronger than those made
predominately
from aluminium and iron.
Is there any way to make suitable magnets without rare
earths?
GH: At the moment there is no way to produce permanent
magnets with the same magnetic output as Nd-Fe-B and Sm-Co,
without using rare earths.
There is some work that has been
initiated recently to look for ways to produce such magnets,
but it will be a tough challenge.
For example, some researchers in
the USA and elsewhere, are looking into using combinations of
Fe and Nd-Fe-B at the nanoscale, to produce strong magnetic
materials. These would still contain rare earths, but less of
them. This is a promising area but the challenges are very
real, particularly in terms of production.
There is also work being done,
mostly in Japan, to try to reduce the overall usage of heavy
rare earth elements such as Dy in Nd-Fe-B magnets, while
maintaining good, higher temperature characteristics. This is
yielding some promising results already.
Note that the milestones in the
development of permanent magnet materials are the result of
significant materials science and engineering efforts.
Understanding the underlying metallurgy, materials science and
alloy chemistry is a critical key to the development of future
materials.
Can we not just create a synthetic product that mimics the
properties of rare earths?
GH: I would say that these magnet materials
are in fact synthetic. The particular combinations of
elements that form the magnetic compounds used in these
magnets, are not found in nature and a lot of special
processing has to be done to optimise the properties, even if
you have the right recipe and all the right ingredients.
Magnets are a significant user of
rare earths. However, there other major uses for rare earths,
particularly for lanthanum and cerium. An example is the use of
lanthanum in the production of fluid cracking catalysts for the
petrochemical industry, used to convert the very viscous, high
molecular weight hydrocarbons found in crude oil, into more
useful, lower molecular weight hydrocarbons.
They are also key in catalytic
convertors for automobiles cerium oxide is combined with
platinum group metals to make these devices work. Lanthanum and
other rare earths can also be found in the alloys used for
nickel-metal-hydride batteries, found in every Toyota Prius and
other hybrid vehicles.
Does the magnet industry have an overreliance on a product
based on a scarce resource?
GH: It depends on how you define
“overreliance”. At present, despite the media frenzy
surrounding rare earths, there are no indications of problems
of supply of finished rare earth permanent magnet materials,
produced in China, to the West.
Recent discussions on export
restrictions relate to the procurement of raw rare earths, and
heavy rare earths in particular, not finished rare earth-based
goods.
However, just because there have
been no real problems to date, obviously this does not mean
that the supply chain is not vulnerable. On the contrary, being
reliant on a single geographic area such as China, regardless
of the geopolitics involved, or even the materials in question,
is clearly a strategic risk.
Security of supply is surely much
more about the prevention of potential problems than it is
about waiting for something bad to happen, and then dealing
with the aftermath via some form of “plan B”.
As such, many companies involved in
the procurement of magnet materials for the production of end
use magnet components and devices, are rightly concerned that
the sources of supply are not diverse enough.
This is of course much easier to say, than to actually put
into practice, but needs to be given some serious
consideration.
Gareth Hatch is the founding principal of Technology Metals
Research, LLC based in Chicago, USA. Until recently he was
Director of Technology at Dexter Magnetics Technologies, a
magnetic engineering and manufacturing firm. He still consults
for Dexter on a regular basis.