29-08-2013, 02:56 PM
Magnetic Nanocomposite Materials for High Temperature Applications
Nanocomposite Materials.PDF (Size: 228.12 KB / Downloads: 60)
Abstract
Magnetic nanocomposites comprised of nano-sized
magnetic crystals embedded in an amorphous matrix
have been shown to have excellent soft magnetic
properties. In particular, amorphous and nanocrystalline
materials have been investigated for various soft
magnetic applications including transformers and
inductive devices. The historical development of
nanocomposite soft magnetic materials will be
reviewed. In these materials it has been determined that
an important averaging of the magnetocrystalline
anisotropy over many grains coupled within an
exchange length is at the root of the magnetic softness
of these materials. The crystallization kinetics and the
chemical partitioning occurring during crystallization
will be described.
In particular, the role of the
amorphous phase in exchange coupling magnetic
nanoparticles at elevated temperatures will be
discussed.
Historical Background
Nanocomposite magnetic materials have their
origins in the amorphous alloys that were brought to
market in the 1970's. Amorphous materials are
characterized by a lack of long range atomic order,
similar to that of the liquid state. Production techniques
include rapid quenching from the melt and physical
vapor deposition is another. The lack of crystallinity
causes amorphous materials to have a very low
magnetic anisotropy. METGLAS 2605TM Fe78 Si13 B9 is
a common amorphous magnetic alloy, in which B acts
as a glass forming element.
The importance of anisotropy
suggests
searching for other materials with isotropic magnetic
properties. In magnetic materials the ferromagnetic
exchange length expresses the characteristic distance
over which a magnetic atom influences it's
environment, and has values on the order of 100 nm. If
the magnet has a structure with grain diameters smaller
than the exchange length, it becomes possible to
"average" the anisotropy of the grains to a low bulk
value. This phenomenon of randomized anisotropy will
be discussed in the next section. Such a material then
realizes the high saturation Ms of the crystalline state
and low Hc due to randomized anisotropy.
Randomized Anisotropy
The notion of randomized anisotropy was
used by Harris, Plischke, and Zuckerman [4] to explain
amorphous rare earth-transition metal alloys. The rare
earth atoms have non-spherical electron orbitals with
high anisotropy. In crystalline materials, this leads to
high Hc and hard magnetic behavior. Alben, Becker
and Chi extended the model to amorphous soft magnets
[5]. The lower anisotropy of the soft magnetic atoms
allows the ferromagnetic exchange length to become
larger. This enables a larger volume to be sampled for
randomizing anisotropy. An energy expression can be
written that balances exchange energy vs.
Thermal Particle Decoupling
The amorphous and crystalline phases in
nanocrystalline magnetic alloys have distinct
behaviors. In particular, the Tc of the amorphous
phases can be different. In FINEMET and
NANOPERM alloys, the amorphous phase Tc is
approximately 400 K, whereas the crystalline grains
have their Tc in the vicinity of 1000 K. Such
decoupling could be observed as a peak in the Hc as a
function of temperature. Work by Varga [8] on
NANOPERM and FINEMET has observed decoupling
at temperatures higher than the Curie temperature of
the amorphous phase. This can indicate that the
particles are remaining coupled either by exchange
field penetration of the amorphous phase, or through
dipolar interactions of the particles.
Conclusions
Nanocomposite magnetic materials have been
developed as an evolution of amorphous magnetic
alloys. These alloys display low Hc and power loss due
to randomized anisotropy of the two-phase structure of
nanocrystalline grains embedded in an amorphous
matrix. The HITPERM alloys, based on the FeCo
system, offer high M
s and T
c. HITPERM has been
shown to have the highest interparticle decoupling
temperatures yet observed. These properties show
promise for increasing efficiency and component
packing density in power electronic applications.