Rare-Earth-Free Magnet Made
from Cheap Materials
US
By Steve Bush, Electronics Weekly, May 17, 2016
US
from Cheap Materials
By Steve Bush, Electronics Weekly, May 17, 2016
It is only a tiny sample, a film
500nm thick, but it is the real thing.
“To the best of our knowledge, this
could be the first experimental evidence of the existence of a giant saturation
magnetisation, an obviously large coercivity, with a magnetic energy product of
up to 20 MGOe, in a bulk-type FeN sample.” said the team in ‘Synthesis of Fe16N2 compound
free-standing foils with 20MGOe magnetic energy product by nitrogen
ion-implantation‘, a Nature Scientic Reports paper written by a team
from the University of Minnesota, Los Alamos National Laboratory and Oak Ridge
National Laboratory.
While the elements iron and nitrogen
are simple and well-understood, and the excellent magnetic properties of Fe16N2
have been long-predicted (theoretical BHmax=135MGOe), the material has proved
extraordinarily difficult to make.
This is partly because the desirable
α˝-martensite crystal structure is only stable below 214°C, whist >300°C is
needed to give the material the correct grain microstructure.
By bonding an iron layer to a
silicon wafer, implanting nitrogen into the iron, then heat-treating the
assemby, the researchers have created a nano-structured material, with 25-30nm
grains, in which the desirable α˝-Fe16N2 martensite
structure has been encouraged by introducing strain – strain which is
generated during the post-annealing process by what appears to be the mismatch
of thermal coefficients between iron foil and silicon substrate (although the
paper said it is between iron foil and iron substrate).
It is estimated that the material
has ~35% of Fe16N2, with the rest made from less
desirable Fe4N and an iron-nickel nitride (Fe4−yNixN).
Nickel compounds results from a nickel film deposited on the iron to keep
nitrogen inside during annealing. Some iron silicide also formed.
At the crystal level (see diagram),
both hard magnet Fe16N2 and soft magnet Fe4N
possess N- centered Fe-N octahedral cluster. In Fe16N2 Fe-N
clusters are separated from each other, while in Fe4N Fe-N clusters
share corner Fe atoms.
This project was started six years
ago the US Government’s Advanced Research Projects Agency (ARPA-E), along with
a number of others aimed at reducing reliance on rare earth elements.
Demand for permanent magnets is
increasing as, in the search for higher efficiency and smaller size, they
replace electromagnets in motors and generators. This demand is expected to
rocket as more electric cars and wind turbines are made.
The most powerful, durable and
useful permanent magnets contain either neodymium or samarium – two materials
that are rare in the Earth’s crust – hence the term ‘rare earth’ – although
gold is rarer than almost all rare earths. Most supplies come from China
The environment could also benefit
from rare earth-free magnets.
“Rare earth elements are not really
rare in principle,” project lead Professor Jian-Ping Wang of the University of Minnesota
Wang pointed out that, although his
team has demonstrated free-standing FeN permanent magnet foil, “it still needs
some time to implement a manufacture synthesis process, meanwhile further
improving its energy product,” he said.
Steel (alnico) magnets are available
that are just as powerful as their rare-earth cousins (they have high
‘remanence’), but their usefulness is restricted because they are easily
de-magnetised (their ‘coercivity’ is low: ~51kA/m) compared to rare-earth
magnets (~1MA/M).
It is the existance of
coercivity>0 that differentiates permanent (‘hard’) magnets from ‘soft’
magnetic materials that cannot support a permanent field. Coercivity is so
low in alnico magnets, that their own field can de-magnetise themselves – hence
the need for ‘keepers’ on horseshoe magnets.
A combined measure of remanence and
coercivity is the figure-of-merit ‘BHmax’, measured in MGOe or kJ/m3 for which
Nd, Sm and alnico magnets score 10-48, 16-33 and 5.5MGOe respectively.
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