chaPter 7 nanomaterials: Properties
250
Metal-matrix nanocomposites can also be beneficial for electrical
contacts. For example, Ag-based nanocomposites filled with SnO 2
nanoparticles, fabricated during a reactive milling process, were hotpressed into electrical contacts. These exhibit outstanding thermal
and electrical conductivity while maintaining superior wear resistance. Metal-matrix nanocomposites are also particularly useful for
magnetic applications such as magnetic recording and giantmagneto resistance. Typically, these nanocomposites are composed of
nanoscale hard magnetic particles such as Nd 2 Fe 14 B Sm 2 Fe 17 N 3 and
FePt, embedded in a soft magnetic nanocrystalline phase, such as
ferrite and Fe 3 Pt. These hard and soft magnetic phases interact magnetically, combining the best properties of a soft magnetic phase,
which is the high saturation magnetization, with that of a hard
phase, the high coercive field (applied magnetic field required to
reduce the magnetization of that material to zero after the magnetization has been brought to saturation). For the interaction to
occur and exert its coupling effect, the two phases must be at the
nanoscale.
In the absence of a magnetic field, the magnetic interaction results
in spin alignment, but when a magnetic field is applied in the
opposite direction (if not above a critical value), only the soft phase
is able to reverse the magnetization. As a consequence, when the
magnetic field is removed the magnetization is again reversed in
the soft phase. However, when the applied magnetic field is high
enough to reverse the spins in the hard phase, the soft phase does
not reverse magnetization when the field is removed. This effect is
strongly dependent on the size of particles as well as the volume
fraction and distribution of each phase. Following this type of interaction, nanocomposite materials exhibit high remanence (magnetization that is left behind after the magnetic field has been removed)
and a high magnetic energy (as high as 200 kJ/m
3 ). An important
aspect is that of maximization of the soft-phase content to enhance
the saturation magnetization. These materials have been processed
through various methods, although the most successful route has
been mechanical alloying of two phases.
In addition to the use of nanoparticles, metal matrix-nanocomposites have also been reinforced with CNTs. Al-matrix nanocomposites reinforced with MWCNTs are such an example. These were
produced by mechanical milling and powder metallurgy. The yield
stress, maximum strength, and hardness values obtained for the
nanocomposites were considerably higher than those for pure Al.
In fact, the addition of 0.75 wt% of MWCNTs has doubled the yield
strength of the nanocomposite.
250
Metal-matrix nanocomposites can also be beneficial for electrical
contacts. For example, Ag-based nanocomposites filled with SnO 2
nanoparticles, fabricated during a reactive milling process, were hotpressed into electrical contacts. These exhibit outstanding thermal
and electrical conductivity while maintaining superior wear resistance. Metal-matrix nanocomposites are also particularly useful for
magnetic applications such as magnetic recording and giantmagneto resistance. Typically, these nanocomposites are composed of
nanoscale hard magnetic particles such as Nd 2 Fe 14 B Sm 2 Fe 17 N 3 and
FePt, embedded in a soft magnetic nanocrystalline phase, such as
ferrite and Fe 3 Pt. These hard and soft magnetic phases interact magnetically, combining the best properties of a soft magnetic phase,
which is the high saturation magnetization, with that of a hard
phase, the high coercive field (applied magnetic field required to
reduce the magnetization of that material to zero after the magnetization has been brought to saturation). For the interaction to
occur and exert its coupling effect, the two phases must be at the
nanoscale.
In the absence of a magnetic field, the magnetic interaction results
in spin alignment, but when a magnetic field is applied in the
opposite direction (if not above a critical value), only the soft phase
is able to reverse the magnetization. As a consequence, when the
magnetic field is removed the magnetization is again reversed in
the soft phase. However, when the applied magnetic field is high
enough to reverse the spins in the hard phase, the soft phase does
not reverse magnetization when the field is removed. This effect is
strongly dependent on the size of particles as well as the volume
fraction and distribution of each phase. Following this type of interaction, nanocomposite materials exhibit high remanence (magnetization that is left behind after the magnetic field has been removed)
and a high magnetic energy (as high as 200 kJ/m
3 ). An important
aspect is that of maximization of the soft-phase content to enhance
the saturation magnetization. These materials have been processed
through various methods, although the most successful route has
been mechanical alloying of two phases.
In addition to the use of nanoparticles, metal matrix-nanocomposites have also been reinforced with CNTs. Al-matrix nanocomposites reinforced with MWCNTs are such an example. These were
produced by mechanical milling and powder metallurgy. The yield
stress, maximum strength, and hardness values obtained for the
nanocomposites were considerably higher than those for pure Al.
In fact, the addition of 0.75 wt% of MWCNTs has doubled the yield
strength of the nanocomposite.