Chemistry Reference
In-Depth Information
superparamagnetic colloids. The critical radius r
c
for a spherical particle with
the stability defined by the flipping probability of the magnetic moment of
o
10% over one second can be estimated using the following equation:
Þ
1
=
3
r
c
¼
6k
B
T
=
K
u
ð
(2
:
2)
d
n
3
r
4
n
g
|
7
where k
B
is the Boltzmann constant, T is temperature, and K
u
is the
crystalline magnetoanisotropy. Depending on k
u
, the critical radii of nano-
particles can be 3-4 nm for very hard magnets and over ca. 20 nm for soft
magnets.
104
Superparamagnetic colloids have great potential for many other
applications related to biomedical research.
104
They have found widespread
use in many traditional areas including magnetic data storage, ferrofluid
technology, magnetorheological polishing, and energy storage. To this end,
superparamagnetic colloids have been exploited for labeling and separation
of DNA, proteins, bacteria, and various biological species, as well as being
applied to magnetic resonance imaging (MRI), guided drug delivery, and the
hyperthermia treatment of cancer. Even certain types of molecular
interactions can also be probed in vivo using specially designed magnetic
probes.
105
In recent years, a wealth of magnetic materials have been
prepared as superparamagnetic colloids using a variety of chemical methods.
In addition to the aqueous system, there is rapid progress in the synthesis of
superparamagnetic nanoparticles using nonhydrolytic solvents.
106-108
Differ-
ent precursors, solvents, and capping agents have all been systematically
examined for producing monodisperse superparamagnetic particles.
.
2.3 Summary
The properties of nanomaterials are substantially different from those of
their bulk counterparts. In this chapter, the unique characteristics of
nanomaterials including thermal, electrical, phonon transport, mechanical,
optical and magnetic properties were discussed.
The unique characteristics arise from many different aspects, for example,
the huge surface area of the nanomaterial is responsible for the reduction in
thermal stability and the supermagnetism; the increased surface scattering is
responsible for the reduced electrical conductivity; size confinement results in
a change of both electronic and optical properties; and the reduction in size
decreases the defects and increases the perfection and mechanical properties.
Hierarchical nanostructures still hold the unique characteristics of the
nanomaterials and they can even be amplified by smart design of the
functional nanostructures. Especially, the hierarchical nanostructures are
designed to possess larger surface areas with enhanced electrical transport
characteristics for fast carrier transport. That is the reason this topic focuses
more on the energy devices than any other applications such as bio-
engineering. All of the energy devices explained in this topic will basically be
based on the unique characteristics explained in this chapter.
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