Biomedical Engineering Reference
In-Depth Information
nanofluids can be synthesized by a one-step process is given in Table 11.1.
Most of the materials used as dispersoids have been metals and their oxides
as they are easy to produce and chemically stable in solution. The base fluids
include water, ethylene glycol, several types of oils, toluene, dielectric fluids,
ethanol, etc.
Early reports show the possibility of synthesis of silver-silicone oil
nanofluid synthesis following a VEROS technique (Yatsuya et al., 1978). In
this method, metals were evaporated in vacuum onto the surface of running
oil, and thus fine particles in the nanometer range were grown on the surface
of the oil. Later, Wagener and Gunther (1999) used a modified VERL
technique employing high-pressure magnetron sputtering to develop iron-
and silver-silicone oil nanofluids. The particles formed by this technique
formed agglomerates. To stabilize the fluids, different dispersants were used.
The mean sizes of the particles for Fe nanofluids were 15 nm (without
surfactant) and 9 nm (with surfactant). The size for Ag nanofluids varied
from 5 to 15 nm by varying the pressure of the system. In another method,
Eastman et al. (2001) employed a direct evaporation condensation (DEC)
technique to synthesize copper-ethylene glycol nanofluids. This technique
involves the vaporization of a source material to be dispersed into the fluid
under vacuum conditions in a chamber containing the base fluid, which is
rotated, and a thin film of the fluid is constantly being transported over the
top of the chamber. Advantages of this technique are that nanoparticles are
produced without oxide layers, the size of the nanoparticles is in a narrow
range, and nanofluids without particle agglomeration can be produced. On
the contrary, the disadvantages are that the liquid must have a very low
vapor pressure and this technique can only produce very limited amounts of
nanofluids.
In another widely used one-step process, nanofluids were prepared by the
chemical reduction method (Patel et al., 2003; Zhu et al., 2004; Zhang et al.,
2006a; Liu et al., 2006; Fuentes et al., 2008; Mishra et al., 2009; Wang and
Wei, 2009). In this technique, a precursor solution is chemically reduced by
a reducing agent to produce nanoparticles in suspension under boiling
conditions. In SANSS, a solid bar of the particle to be dispersed, submerged
in the base fluid, is used as an electrode in a vacuum system which melts and
vaporizes in the region of high-temperature electrical arc generated by the
system. The base fluid medium also vaporizes and rapidly removes the vapor
of the solid and cools it to restrain further particle growth. Nanoparticles
are then formed from the evaporated solid and are well dispersed in the
cooling medium through three transformation stages, namely nucleation,
growth, and condensation (Lo et al., 2005a, 2005b). Alternatively, silver-
water nanofluids were prepared by multi-pulse laser ablation technique
where a silver bar submerged in de-ionized water was ablated using a
double-beam approach (Phuoc et al., 2007).
￿ ￿ ￿ ￿ ￿ ￿
Search WWH ::




Custom Search