Civil Engineering Reference
In-Depth Information
hours. h e precipitated particles were then oxidized in ambient atmosphere at 80°C for
2 hours to form ferrite particles dispersed in BC network. h e size of the particles was
found to have a bimodal distribution with 20 nm and 70 nm as the two average sizes.
h e saturation magnetization increases with increasing precursor solution concentra-
tion as well as increased soaking time (Figure 17.4 a and b). A maximum of 28 emu/g
saturation magnetization was realized by soaking the BC micro-pellets for 4 hours in
0.1 mol/l Fe
2+
precursor solution. h is method, however, does not result in the forma-
tion of a magnetic sheet but a magnetic suspension. It can be made into a magnetic
paper by casting this suspension onto a i lter paper and then drying it in air. In order
to control the particle size as well as its distribution in the BC network, Zheng
et al.
[34] sonicated the precursor-loaded BC during reduction in NaOH solution and they
also added PEG (Poly ethylene glycol) a surfactant, to the NaOH solution. It was found
that sonication and sonication in the presence of a surfactant leads to more uniform
particle size as well as their distribution (Figure 17.5 a, b and c). h e freeze-dried com-
posite sheet was found to be highly l exible and bendable by 360°. h is results in the
formation of a magnetically actuating sheet. In both the methods mentioned above the
reducing agent, NaOH, was in the solution form and this can reduce its ability to per-
meate deep into the BC network. Hence Kateptech
et al.
[35] used ammonia gas as the
reducing agent to co-precipitate ferrite nanoparticles. h ey found that the size of the
ferrite nanoparticles increased from 19.6 nm to 39 nm on increasing the concentration
of Fe precursor. h e uniform distribution of the ferrite particles and bending and actu-
ating capability of the composite sheet by external magnetic i elds were demonstrated.
h e problem with these magnetic actuators, however, is that they are amphiphilic, i.e.,
hydrophilic-lipophilic, which makes them water absorbing. h is leads to dimensional
instability and a lower performance as water absorption leads to increased weight and
lower ef ective magnetization. In order to overcome this behavior, Zhang
et al.
[36]
coated the BC i bers with amphiphobic l uroalkyl silane to reduce water absorption
capacity. h ey found that Fe
3
O
4
nanoparticles containing BC were more ef ective in
being amphiphobic on coating l uroalkyl silane compared to pure BC. h e presence
of Fe
3
O
4
nanoparticles increases the ef ective roughness and thus increases the wetting
angle from 81° and 67° to 130° and 110° for water and oil respectively when coated with
the silane compound (Figure 17.6a). h e magnetic actuating capability of the coated
Fe
3
O
4
-BC composite was found to be strongly present even at er coating with silane
( Figure 17.6b ). h ese magnetic composites were also found to ef ectively shield the
electromagnetic radiation [37] .
h e Fe
3
O
4
-BC magnetic composites synthesized by aqueous phase reduction are in
general superparamagnetic at room temperature with relatively low saturation magne-
tization and no coercivity. Incorporation of hard magnetic particles will lead to the for-
mation of a magnetic composite that can store/deliver large amounts of energy. Olsson
et al.
[38] synthesized l exible magnetic aerogels and xerogels by incorporating a rela-
tively hard magnetic material such as CoFe
2
O
4
. h e freeze-dried BC was i rst soaked in
an aqueous mixture of FeSO
4
and CoCl
2
and then reduced using a mixture of NaOH
and KNO
3
solution at 90°C for 3 hours. h e resulting CoFe
2
O
4
-BC hydrogel was freeze
dried to obtain a l exible aerogel. h is aerogel could be mechanically compressed to
obtain a relatively stif magnetic paper. h ese composites were found to exhibit a hys-
teretic magnetic behavior at room temperature with saturation magnetization as high
