Carbon Nanotubes Reinforced Cement Composites - Applications & Performances

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Chapter I CARBON NANOTUBES 
 
 
 
1 THE DISCOVERY OF CARBON NANOTUBES 
 
In 1985 the American chemist Richard E. Smalley discovered that, in particular 
situations, the atoms of carbon compose some orderly structures of spherical form: the 
fullerenes. The structure, after a following loosening, tends to roll up on itself, getting a 
typical cylindrical form: the carbon nanotubes (CNT). They can likewise be seen to the 
fullerene as one of the carbon allotropic forms. 
In the 1991 Sumio Iijima, at the NEC Fundamental Research Laboratory of Tsukuba 
(Japan), used a high resolution transmission electron microscope (TEM) in order to 
study the particulate created in an electric discharge between two carbon electrodes. He 
discovered this particulate contained structures consisting of a lot concentric carbon 
tubes. 
One year later, Thomas Ebbesen and Pulickel Ajayan, also they at the NEC in Tsukuba, 
developed an efficient method to manufacture great quantities of these so-called Multi 
Wall NanoTubes (MWNTs). 
Subsequently, in 1993, the Iijima group, at the NEC, and the Donald Bethune, at the 
IBM Almaden Research Center in California, independently discovered the Single Wall 
NanoTubes (SWNTs). 
In the last decade, a lot of researches about carbon nanotubes began. In the few years 
from their discovery innumerable interesting characteristics and many potential 
applications are individualized. 
 
1.1 FULLERENES  
 
Among the most promising substances for the possible nano-technological implications 
must surely be remembered the crystalline carbon in its various forms. Till 1985 only
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two forms were known: the three-dimensional form of the diamond (sp
3
) and the bi-
dimensional form of the graphite (sp
2
). 
The studies made by Robert F. Curl and by Richard E. Smalley of the Rice University 
(Texas) and by Harry Kroto of the University of the Sussex brought to the discovery, in 
1985, of a third form of regular arrangement of the carbon atoms: the fullerenes. Thanks 
to this discovery they got the Nobel Prize in 1996. 
The fullerenes are approximately spherical “cages” formed by an orderly arrangement 
of hexagonal and pentagonal structures of carbon atoms. The amount of present 
polygons and their relative proportion determine the form and the dimensions of the 
fullerene. The first discovered fullerene was the C
60
. It is a sphere formed by 60 atoms 
of carbon with a diameter of around 1 nm. It is entirely similar to a common kick ball. 
For this reason it is also known with the name of “buckyball” (Figure 1). 
This family has taken the name of “fullerenes” in honour of the architect Richard 
Buckminster-Fuller. Indeed his creations, called “geodesic domes”, remember the 
structure of the fullerenes (Figure 2). 
 
 
Figure 1 - Fullerene C
60
.[F1]
 
 
Figure 2 – Dome made by Richard 
Buckminster-Fuller.[F1]
 
The fullerene is the third form of the carbon as well as the graphite and the diamond. 
In combination with some elements fullerenes can form superconductor crystals with 
interesting characteristics. In presence of oxygen and light they promote particular 
oxidation reactions towards some types of organic molecules (e.g., the DNA). It could 
find applications in pharmacological field or in the disposal of polluting residues. Their 
photo-physical properties make them a good candidate to the realization of devices of 
optic limitation. Among those there are smart transparent materials stopping to a certain
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threshold beams of light with superior intensity (e.g., protecting the eyes of whom 
works with the lasers). Particular interest has aroused the discovery of C
60
 ability to 
bond with an enzyme of the virus HIV stopping its replication. This happens because of 
form, dimensions and chemical characteristics of fullerenes. To this day this function 
has been found only in vitro. 
The fullerenes are artificially produced with a system of graphite vaporization by 
electric discharges or laser impulse. Fullerenes have also been found in least 
percentages even in the Yinpinglang coal mine (China). 
 
1.2 CARBON NANOTUBES 
 
The nano-fibres are fibrous structures whose diameter is between some about ten and 
some hundred nanometres. These fibres can have got very different structures. Those 
are the “graphite whiskers” (constituted by a layer of graphite rolled up more times on 
itself) and the fibres “platelet” (constituted by layers of graphite perpendicular to the 
axle of the fibre). Nano-fibres can be divided into three great families, according to the 
formed angle between the filament axle and the graphite layers plan. 
However, “standard” nano-fibres don’t exist and the terms and the definitions used in 
the published papers exclusively depend on the personal authors choices. 
Carbon NanoTubes (CNTs) are formed by a graphite sheet (that is a hexagonal network 
of carbon atoms) rolled up in the shape of pipe (Figure 3). The two extremities are 
closed by hemispherical caps. The diameter is generally between 1 nm and 2 nm, 
corresponding to a ring composed by thirty hexagons of carbon atoms. The length can 
be extended for different microns. The external and inner specific surfaces are in the 
order of magnitude of 400 m
2
/g and 300 m
2
/g respectively. 
Extremities of CNTs are formed by pentagons that support the bending. However 
following papers have shown that it isn’t necessary that domes are conic or 
hemispherical, but they can simply form oblique structures. Just for this conformation 
of hexagons and pentagons, the carbon nanotubes often show some structural defects or 
imperfections deforming the cylinder.
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Figure 3 – Carbon nanotube picture.[F2] 
 
CNTs have a high tensile strength up to about 100 times than steel, despite their specific 
weight is six times lower. From their first synthesis in 1991, carbon nanotubes have 
been employed in different fields, as electronics, biology, chemistry and particularly in 
preparation of composite materials. Above all the CNTs have been used to reinforce 
polymers, metals and ceramic materials. 
 
 
2 TYPOLOGIES OF CARBON NANOTUBES 
Carbon nanotubes can be divided into two categories: Single Wall carbon NanoTubes 
(SWNTs), if they are constituted by only one graphite sheet, and Multi Wall carbon 
NanoTubes (MWNTs) if they are formed by some sheets positioned as concentric 
cylinders inserted one inside the other. In the MWNTs every single nanotube maintains 
its properties. Thus it is very difficult to predict the resultant behaviour. Besides these 
multi wall carbon nanotubes usually contain a great number of defects and this limits 
their possibilities of employment.
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2.1 SINGLE WALL CARBON NANOTUBE – SWNT 
 
SWNTs have been firstly produced in 1993 using an electric arc system with electrodes 
composed by a mixture coal-cobalt. An ideal SWNT can be described as a carbon tube 
formed by a graphite sheet rolled up on itself in order to form a cylinder. The two ends 
are closed by two hemispherical caps (Figure 4). 
 
 
Figure 4 - Ideal SWNT.[F1] 
 
The structure of the single wall carbon nanotube is formed of only hexagons. The caps 
(two hemisphere) are formed by hexagons and pentagons, and they correspond to two 
half molecules of fullerene. 
In the reality the carbon nanotubes often show structural defects or imperfections of the 
geometric structure (e.g., the presence of pentagonal or heptagonal structures in the 
body of the tube). These structures deform the cylinder. 
The diameter of a SWNT ranges from 0.7 nm (correspondent to the double of the 
distance between two graphite sheets) to 10 nm. However, in the majority of the cases, 
the diameter is less than 2 nm. 
The high aspect ratio (10
4 
÷ 10
5
) of the SWNTs allows considering them as virtually 
mono-dimensional nanostructures. It confers to these elements peculiar properties. 
 
2.2 MULTI WALL CARBON NANOTUBE –  MWNT 
 
The MWNTs are carbon nanotubes formed by more concentric SWNTs (Figure 5).
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In the MWNTs, bonds can be present among each wall (lip-lip interactions) that it 
seems to stabilize the growth of these carbon nanotubes. 
The diameter of MWNTs is greater than SWNTs. It increases with the number of walls, 
also reaching tens of nanometres. 
The difference between MWNTs and nano-fibres is not very well defined. Indeed a 
MWNT of great dimensions can be considered as a particular case of tubular nano-fibre. 
A great number of structural defects or interactions between walls inside the tube is 
possible, thus it make even more fleeting the distinction between these structures. 
 
 
Figure 5 – Computer pictures of DWNTs (Double Wall carbon NanoTubes) 
with and without alterations between walls.[F1] 
 
The MWNTs often have a great number of imperfections in their structure. They also 
show an extreme variety of shapes in their terminal zone. 
[1]
 
 
 
3 CHARACTERISTICS OF CARBON NANOTUBES 
 
Every SWNT is characterized by its diameter and its “chiral vector” [n,n] or “helicity”, 
that is the rolling up direction of the graphite sheet in relationship to the axle of the tube 
(Figure 7).
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According to the rolling up sense, carbon nanotubes can be divided into three types. If 
the meshes are arranged with two sides of hexagons parallel or perpendicular to the 
nanotube axle, CNTs are called “zigzag” or “armchair” respectively (according to the 
profile that atoms draw in a nanotube section perpendicularly to its axle). If the sides of 
the hexagons are progressively staggered and they determine the spiral movement, 
carbon nanotubes are called “chirals” (Figure 6).  
 
 
Figure 6 - Three different typologies of nanotubes.[F1] 
 
Therefore, the structures observed by Iijima can be visualized as a bi-dimensional 
graphitic network on a cylindrical surface. Each vector R defines a different way to roll 
up the sheet in order to form the tube. If we introduce the unit vectors R
1
 and R
2
, R can 
be express as: 
 
2 2 1 1
R n R n R  +  = 
 
and each tube can be identified with a couple of integers [n
1
, n
2
] (Figure 7).