Environmental Engineering Reference
In-Depth Information
ment of performance can be achieved despite some agglomeration of the nanopar-
ticles. However, this might be improved further if a macroscopically homogeneous
dispersion could be prepared. The issue of dispersion is closely related to that of
reactivity in that the search for a better capping agent to prevent further reaction
at the surface once the nanoparticle has been prepared must also meet the need
to produce stable dispersion of nanoparticles. In addition to this, it is advantageous
if the dispersions remain stable at a range of concentrations, so that very concen-
trated dispersion of nanoparticles may be prepared and then processed in order to
prepare the fi nal material or device.
2.5.4
Cost
The preparation of some nanoparticles requires the use of very expensive reagents
and solvents. This, in many cases, increases the cost and complexity of scale up
signifi cantly. The need to fi nd low cost, simple routes to materials is key in scaling
up the preparation of certain products.
2.5.5
Methods: Natural Sources
Perhaps the simplest way to obtain a nanomaterial is to use a raw mineral with a
minimal amount of post processing. A few commercial nanomaterials are prepared
by simple processing of minerals. For example, asbestos has six main forms
(Table 2.4), of which chrysotile, amosite and amphibole were the main forms. They
are employed in applications along with small amounts of termolite. Asbestos fi bres
are silicate-based minerals and are either magnesium or iron silicates. They have
a highly anisotropic wire-like structure with diameters in the nanometre range and
lengths of several microns. It is well known now that asbestos and other fi brous
materials with similar length scales are very harmful to human heath (Castleman,
2006) and are no longer in widespread use in the many countries.
Another form of nanomaterial which has found commercial application is the
so called nano-clays. These are a post processed form of layered silicates such as
montmorillonite. These minerals have a layered structure with intercalated metal
ions which serve both to balance the charge on the silicate plates and to bind the
layers together. These layers may be separated by exchanging the metal ions for
Table 2.4 Forms of asbestos, their common names and formulas. (Other names may be
used and can be found in Nolan et al., 1999)
Name
Mineral group
Asbestos type
Formula
Chrysotile
serpentine
white
Mg 3 Si 2 O 5 (OH) 4
Actinolite
amphibole
Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2
Tremolite
amphibole
Ca 2 Mg 5 Si 8 O 22 (OH) 2
Grunerite
amphibole
brown
(Fe (5-7), Mg (2-0) ) Si 8 O 22 (OH) 2
(Fe 2+ , Mg) 7 Si 8 O 22 (OH) 2
Anthophyllite
amphibole
grey
Na Fe
Riebeckite
amphibole
blue
(
)
+
22
3
Fe
2
+
,
Mg
Si O
(
OH
)
8 2
2
3
 
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