Literature Review
Supersoft Magnets made from Nanoscale dispersion of Fe-Co in
Ceramic Matrix
R. Raj, Atanu Saha and Masood Hasheminiasari
Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309
Soft Magnetic Materials
Soft or high permeability magnetic materials are defined as those in which very large
changes in the magnetic flux density can be produced by very small fields, sometimes as
low as 8 A/m. The most convenient parameter for describing a soft magnetic material is
the permeability μ= B/H, where B is the flux density produced by the applied field H.
Since flux density is related to the magnetization of the material, for high permeabilities,
we require materials with high saturation magnetization and very low coercivities, so that
large changes in the magnetization, occurring either by domain wall displacement or
ideally very high permeability. Since the coercivity must be as small as possible, soft
magnetic materials with high permeabilities have very narrow hysteresis loops. High
permeabilities can be obtained only if the coercivity is as low as possible, and since this
is highly structure sensitive magnetic parameter, it depends on both intrinsic and extrinsic
properties.
Therefore, some of the important factors are the effects of metallurgical structure at
both atomic and microstructural levels, e.g. crystallographic defects, atomic order,
domain size, impurities, the presence of second phases and their dependence on heat
treatment. As decreasing domain size of Fe-Co dispersant (around Nanometer), the
permeability and susceptibility of magnet will be improved. From the Fe-Co phase
diagram, we know that Co is soluble in Fe up to 75% and that the alloys are all BCC at
ambient temperature. At composition close to Fe-50% Co, they can form a simple
ordered BCC superlatice. However, it is found that both initial and maximum
permeability have their maximum values at about this composition1.
In nanocrystalline state it is thought that the high magnetic permeabilities are obtained
because the magnetocrystalline anisotropy is very much reduced due to local
randomization so that there is no distinct easy direction of magnetization. However, the
coercivities and hence the permeabilities are strongly dependent on the grain size. Herzer
and Warlimont (1992) have pointed out that even for same alloy the coercivity may vary
from 1.0 A/m to 5000 A/m for grain and domain sizes ranging from 10 nm to 40 nm, thus
hard and soft magnetic properties can be observed in the same alloy by changing in grain
size or domain size of particles.
In this project we are trying to achieve the magnets from Nanoscale dispersion of FeCo in ceramic matrix, this involves increasing in permeability or hysteresis curve slope to
get the best magnetic and mechanical properties for this type of ceramic magnets.
Polymer derived Ceramics (Composites)
Preceramic polymers offer a unique method to fabricate ceramic matrix composites
from the polymer roots2. Polymer derived silicon carbonitride exhibits excellent flexural
strength and resistance to creep3, oxidation4, and thermal shock5. It also possesses high
chemical stability at elevated temperatures. In addition to structural properties, polymer
derived ceramics also exhibit high temperature semiconductivity. Their electrical
conductivity can be changed from approximately 1 to 108 Ω-1 cm-1 by doping.
The new direction for polymer derived ceramic has bee reported in previous reports and
papers, that the design and processing of composite with an unusual set of attributes,
incorporating the outstanding structural properties of SiCN as well as the functional
property of the dispersed phase. The dispersed phase may be a metal or ceramic. It is
shown that dispersed iron particles can be introduced by the polymer route, creating a
high-temperature magnetic material for applications in harsh environments. In this
project the nanodispersion of iron-cobalt (Fe/Co) particles in polymer derived ceramic
with Zro2 route, will be studied to gain desired mechanical and magnetic properties for
this supersoft ceramic magnet.
It has been shown that, the processing of composite takes advantage of the polymer
route for the fabrication of SiCN. In this process an organic liquid precursor is
crosslinked and pyrolyzed to produce the SiCN ceramic matrix.
In one of the previous projects they have prepared the SiCN-Fe composites by
incorporating Fe3O4 powder into the liquid precursor (Ceraset, KION Corporation) and
the reducing the ferrite to α-iron during pyrolysis. The powder was dispersed in Ceraset
and the slurry was ultrasonicated to break up the agglomerates, and then the specimens
were pyrolyzed up to different temperatures in nitrogen gas for nine hours, to study the
influence of the pyrolysis temperature on phase evolution. The x-ray diffractogram shows
the composites, pyrolyzed at different temperatures. The pyrolyzed sample shows a
higher susceptibility but a lower saturation magnetization than the other samples. The
difference in magnetization can be explained by the lower magnetization of Fe3O4
relative to α-Fe. The mechanical properties of this ceramic can increase to an average
value of 6.5 GPa in the sample pyrolyzed at 1000˚C. These results show that the
composite pyrolyzed at 1000˚C exhibits the highest density and hardness with best
magnetic properties. The results show that the susceptibility decreases when Fe3O4 is
reduced to α-Fe, there are two possible explanations for this behavior: first one is
magnetostriction, that is, the mechanical constraint imposed on the iron particles by the
SiCN matrix restricts domain rotation and second one is interfacial frustration, that is, the
interface between the particles and the matrix may pin the magnetization and therefore
resist the rotation and motion of the magnetic domains. Both mechanisms can act
together to lower the susceptibility of the composite6.
Processing and Characterization on Fe/Co Nano particles in ZrO2 Ceramic Matrix
The amorphous powders are produced by chemical reduction technique that is shown
below:
FeCl3/CoCl2 + 2NaBH4 →→→ 2NaCl + Fe/Fe-O/Fe, Co/Co-O/Co-complex
First FeCl3 and CoCl2 are dissolved in deionized water and small amount of catalyst is
added as dispersant. The addition of sodium hydro borite reduces the FeCl 3/CoCl2 into
the complex that mentioned above. This reaction is highly exothermic and needs to be
done in controlled way. Crystallization of these powders is done by heat treating the
powder. Heat treatment at 850˚C produces only metal (no oxide is detected by wide angle
x-ray), the size of these nano-particles can be measured by x-ray and TEM.
References
1. R. A. McCurrie. Ferromagnetic Materials Structure and Properties, Academic
Press, London, 1994.
2. R. Jones, A. Szweda, D. Petrak. Polymer derived ceramic matrix composites,
composites: Part A 30 (1999) 569-575.
3. L. An, R. Riedel, C. Konteschny, H-J. Kleebe, and R. Raj, J. Am. Ceramic Soc.
81, 1349 (1998).
4. R. Raj, L. An, S. Shah, R. Riedel, C. Fasel, and H-J. Kleebe, J. Am. Ceramic Soc.
84, 1803 (2001).
5. L. Liew, W. Zhang, L. An, S. Shah, R. Lou, Y. Liu, T. Cross, K. Anseth, V.
Bright, J.W. Daily, and R. Raj, J. Am. Ceramic Soc. Bull., 80, 25 (2001)
6. Atanu Saha, Sandeep R. Shah, and Rishi Raj. Polymer derived SiCN composites
with magnetic properties. J. Mater. Res., Vol. 18, No. 11, Nov 2003.