Water/zinc oxide nanofluidZnO aqueo

Water/zinc oxide nanofluid
ZnO aqueous colloidal suspensions both polygonal and rod-like
nanoparticles came from commercial sources, respectively, from
Nyacol (SN15ES) and Evonik (VP DISP ZnO 20 DW). The colloidal
suspensions were dialyzed in cellulosic membranes (MWCO:
14,000 Da) for 1 week against deionised water in order to remove
all the organics and salts. The efficiency of the dialysis step was
monitored by conductivity measurements of the buffer water.
Finally the solid percentages of nanoparticles in the nanofluids
were adjusted by evaporation or dilution. The nanoparticles were
characterized by transmission electron microscopy (TEM) performed
on a JEOL 2000FX or on a High Resolution Scanning
Transmission Electron Microscope (HRSTEM) Titan.
Contrary to the SiO2 nanoparticles it appears that the polygonal
nanoparticles are massive as observed by means of HRSREM. It was
also observed that they are perfectly crystalline (Fig. 2(a) and (b)).
The mass concentration was 4.4% for Nyacol and 5% for
Evonik based suspensions (volume concentration: 0.82% and 0.93,
respectively).
For both nanofluids, the surfactant effect is a consequence of the
preparation mode of the nanoparticles. A monolayer of repellent
molecules is formed at the particle surface. Due to the thinness of
the layer, its effect is negligible on r and Cp values. Nevertheless the
mass density and the specific heat capacity have been experimentally For SiO2(Ag) particles, hydrated hydrazine is used to reduce
silver salts (silver nitrate), the formed silver d0 metal nanoparticles
will interact with free thiol groups which wrap the silica nanoparticles
and so will form a continuous silver layer at the surface of
the silica nanoparticles. Finally the colloidal silica sols are dialyzed
against water to remove all the water-soluble by-products.
Concerning the commercial water/zinc oxide nanofluids are
dialyzed (MWCO 14KD-30KD) against water to remove all the
water-soluble by-products (biocides, salts, surfactant).
So in the both cases, we can assume that the nanofluids are
clean and that the influence of the remaining organics or salt on the
thermal properties is poor.