Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 48

Table 2.3 Various values for the modulus and strength for a range of materials in comparison
to reported values for carbon nanotubes.

Material Density (gcm − 3 ) Strength (MPa) Modulus (GPa) Reference

Acrylic 1.2 55-70 2.5-3.5 a

E-glass (glass fi bre) 2.5 1 000 77 b

Mild Steel 7.7 340 200 b

Tungsten 19.4 120 410 b

Tantalum 16.6 1 240 186 b

Carbon Fibre 1.6 1 500 130 c

MWCNT (high values) ∼ 1.9 68 000 950 d

MWCNT (low values) ∼ 1.9 1 400 68 e

References: a: Harrison, 1984; b: Thostensona et al. , 2001; c: Performance Composites Ltd, 2008; d: Yu et al. , 2000;
e: Leuenberger and Loss, 2000.

as the properties of a composite are directly related to the properties of the rein-
forcing fi bres and their volume fraction. This also makes carbon nanotubes interest-
ing because they have such low densities compared to conventional materials with
similar mechanical properties. However, there are still several technical challenges
in translating the superior properties of the carbon nanotubes into a high perfor-
mance composite (Thostensona et al. , 2001 ).

2.4.5 Magnetic Properties

There is currently a strong drive to develop nanoparticles with magnetic properties
for a range of applications including data storage. This need for smaller particles is
simply driven by the need to improve the density of data which may be stored on
the same area of a disc. Reducing the particle dimensions by half theoretically
increases the storage density by four times. Materials which exhibit permanent
magnetic properties arise due to the cooperative interactions of electron spins on
individual atoms. In order for a material to exhibit permanent magnetic properties,
that is it can be magnetised and retains its magnetisation once the fi eld has been
removed, the strength of coupling between the spins in the material must be strong
enough such that they are not randomised by thermal processes at room tempera-
ture. Quantisation of the relaxation process has been observed in molecular species
(Leuenberger and Loss, 2000) and it is well known that the barrier to relaxation is
related to the dimensions of the nanoparticle as well as its composition (Guardia
et al. , 2007). The ability to align spins which later relax back into a disordered and
therefore non-magnetic state is called superparamagnetism. The fi eld of magnetic
materials is very large and too complex to deal with in detail here; however, there
are some excellent reviews on the area (Duguet et al. , 2006 ; Xu et al. , 2007 ).

2.4.6 Interfacial Properties

It is well known in colloid science that charge stabilised particles can be used to
stabilise emulsions giving the so-called Pickering emulsions (Aveyard et al. , 2003 ;