Nanoparticles for high performance concrete (HPC) - Nanotechnology in Eco-efficient Construction - page 46

SF particles

3.6 Scanning electron micrograph showing silica fume particles
intermixed with carbon nanofi bers after dry mixing (Sanchez and Ince,
2009).

1. Carbon nanotubes are functionalized in a mixture of nitric and sulfuric

acids (70 wt% and 96 wt%, respectively) at 80°C.

2. Functionalized carbon nanotubes are washed with acetone to remove

carboxylated carbonaceous fragments formed during oxidation of the
nanotubes.

Metaxa et al. (2012) developed an ultracentrifugation concentration
process for the production of highly concentrated suspensions of carbon
nanotubes. Ultracentrifugation is used to reduce the amount of water in the
nanotube water/surfactant suspension, thus increasing the concentration of
nanotubes (Figs 3.7 and 3.8).
The process involves the dispersion of carbon nanotubes in an aqueous
surfactant solution by ultrasonication and ultracentrifugation of the suspen-
sion, followed by decantation and ultrasonication of the remaining suspen-
sion. Absorbance spectroscopy results confi rmed a fi vefold increase in the
concentration of carbon nanotubes in the suspensions. Another important
issue in the dispersion of nanoparticles is its quantitative characterization.
To date, three common methods have been used to analyze the dispersion
of CNTs or CNFs in aqueous solutions. These are optical microscopy, elec-
tron microscopy (using both scanning electron microscopes (SEM) and
transmission electron microscopes (TEM)), and ultraviolet-visible (UV-
Vis) spectroscopy (Tyson et al. , 2011). These authors developed a method
for quantifying the dispersion and agglomeration of both carbon nanofi bers
and carbon nanotubes within an aqueous solution. The dispersion quantity,
D , is measured by the free-path spacing between particles; the agglomera-

￿ ￿ ￿ ￿ ￿ ￿