1. Introduction
Carbon
is a nonmetallic chemical element in Group 14 of the periodic table.
Although widely distributed in nature, carbon is not particularly
plentiful. Elemental carbon exists in several forms, each of which has
its own physical characteristics. Two of its well-defined forms, diamond
and graphite, are crystalline in structure, but they differ in physical
properties because the arrangements of the atoms in their structures are
dissimilar [1, 2]. A third form, called fullerene, consists of a
variety of molecules composed entirely of carbon [3, 4]. Spheroidal,
closed-cage fullerenes are called buckerminsterfullerenes, or
buckyballs, and cylindrical fullerenes are called carbon nanotubes [5,
6]. A fourth form, called Q-carbon, is crystalline and magnetic [7,
8]. Yet another form, called amorphous carbon, has no crystalline
structure. Other forms, such as carbon black, charcoal, lampblack, coal,
and coke, are sometimes called amorphous, but X-ray examination has
revealed that these substances do possess a low degree of crystallinity
[9, 10]. Diamond and graphite occur naturally on Earth, and they
also can be produced synthetically; they are chemically inert but do
combine with oxygen at high temperatures, just as amorphous carbon does
[1, 2]. Fullerene was serendipitously discovered in 1985 as a
synthetic product in the course of laboratory experiments to simulate
the chemistry in the atmosphere of giant stars [3, 4]. It was later
found to occur naturally in tiny amounts on Earth and in meteorites
[5, 6]. Q-carbon is also synthetic, but scientists have speculated
that it could form within the hot environments of some planetary cores
[7, 8]. At ordinary temperatures, carbon is very unreactive, it is
difficult to oxidize, and it does not react with acids or alkalis. At
high temperatures it combines with sulfur vapor to form carbon
disulfide, with silicon and certain metals to form carbides, and with
oxygen to form oxides, of which the most important are carbon monoxide
and carbon dioxide. Because at high temperatures carbon combines readily
with oxygen that is present in compounds with metals, large quantities
of coke are used in metallurgical processes to reduce metal oxide ores.
Pure diamond is the hardest naturally occurring substance known and is a
poor conductor of electricity [11]. Graphite, on the other hand, is
a soft slippery solid that is a good conductor of both heat and
electricity [12]. Carbon as diamond is the most expensive and
brilliant of all the natural gemstones and the hardest of the naturally
occurring abrasives [11]. In microcrystalline and nearly amorphous
form, graphite is used as a black pigment, as an adsorbent, as a fuel,
as a filler for rubber, and, mixed with clay, as the lead of pencils
[12]. Because it conducts electricity but does not melt, graphite is
also used for electrodes in electric furnaces and dry cells as well as
for making crucibles in which metals are melted [13]. Molecules of
fullerene show promise in a range of applications, including
high-tensile-strength materials, unique electronic and energy-storage
devices, and safe encapsulation of flammable gases, such as hydrogen
[14]. Q-carbon, which is created by rapidly cooling a sample of
elemental carbon whose temperature has been raised to 4,000 K [15],
is harder than diamond, and it can be used to manufacture diamond
structures, such as diamond films and microneedles [16], within its
matrix. Elemental carbon is nontoxic.
Each of the amorphous forms of carbon has its own specific character,
and, hence, each has its own particular applications [17, 18]. All
are products of oxidation and other forms of decomposition of organic
compounds. Coal and coke, for example, are used extensively as fuels
[19, 20]. Charcoal is used as an absorptive and filtering agent and
as a fuel and was once widely used as an ingredient in gunpowder [21,
22]. Coals are elemental carbon mixed with varying amounts of carbon
compounds. Coke and charcoal are nearly pure carbon. In addition to its
uses in making inks and paints, carbon black is added to the rubber used
in tires to improve its wearing qualities [23, 24]. Carbon, either
elemental or combined, is usually determined quantitatively by
conversion to carbon dioxide gas, which can then be absorbed by other
chemicals to give either a weighable product or a solution with acidic
properties that can be titrated.
When an element exists in more than one crystalline form, those forms
are called allotropes; the two most common allotropes of carbon are
diamond and graphite [25, 26]. The crystal structure of diamond is
an infinite three-dimensional array of carbon atoms, each of which forms
a structure in which each of the bonds makes equal angles with its
neighbors. If the ends of the bonds are connected, the structure is that
of a tetrahedron, a three-sided pyramid of four faces including the
base. Every carbon atom is covalently bonded at the four corners of the
tetrahedron to four other carbon atoms. The distance between carbon
atoms along the bond is called the single-bond length. The space lattice
of the diamond can be visualized as carbon atoms in puckered hexagonal
rings that lie roughly in one plane, the natural cleavage plane of the
crystal; and these sheets of hexagonal, puckered rings are stacked in
such a way that the atoms in every fourth layer lie in the same position
as those in the first layer. Such a crystal structure can be destroyed
only by the rupture of many strong bonds. Thus, the extreme hardness,
the high sublimation temperature, the presumed extremely high melting
point extrapolated from known behavior, and the reduced chemical
reactivity and insulating properties are all reasonable consequences of
the crystal structure [27, 28]. Because of both the sense and the
direction of the tetrahedral axis, four spatial orientations of carbon
atoms exist, leading to two tetrahedral and two octahedral forms of
diamond.
The crystal structure of graphite amounts to a parallel stacking of
layers of carbon atoms. Within each layer the carbon atoms lie in fused
hexagonal rings that extend infinitely in two dimensions. Each layer
separates two identically oriented layers. Within each layer the
carbon-carbon bond distance is intermediate between the single bond and
double bond distances. All carbon-carbon bonds within a layer are the
same. The interlayer distance is sufficiently large to preclude
localized bonding between the layers; the bonding between layers is
probably by van der Waals interaction, namely the result of attraction
between electrons of one carbon atom and the nuclei of neighboring
atoms. Ready cleavage, as compared with diamond, and electrical
conductivity are consequences of the crystal structure of graphite
[29, 30]. Other related properties are softness, smoothness, and
slipperiness [31, 32]. A less common form of graphite, which occurs
in nature, is based upon an ABCABCA… stacking, in which every
fourth layer is the same. The amorphous varieties of carbon are based
upon microcrystalline forms of graphite. The individual layers of carbon
in graphite are called graphene, which was successfully isolated in
single-layer form.
The greater degree of compactness in the diamond structure as compared
with graphite suggests that by the application of sufficient pressure on
graphite it should be converted to diamond. At room temperature and
atmospheric pressure, diamond is actually less stable than graphite. The
rate of conversion of diamond to graphite is so slow, however, that a
diamond persists in its crystal form indefinitely [33, 34]. As
temperature rises, the rate of conversion to graphite increases
substantially, and at high temperatures it becomes thermodynamically
favorable if the pressure is sufficiently high. At the same time,
however, the rate of conversion decreases as the thermodynamic
favorability increases. Thus, pure graphite does not yield diamond when
heated under high pressure, and it appears that direct deformation of
the graphite structure to the diamond structure in the solid state is
not feasible. The occurrence of diamonds in iron-magnesium silicates in
the volcanic structures called pipes and in iron-nickel and iron sulfide
phases in meteorites suggests that they were formed by dissolution of
carbon in those compounds and subsequent crystallization from them in
the molten state at temperatures and pressures favorable to diamond
stability [35, 36]. The successful synthesis of diamond is based
upon this principle. The crystal structure of graphite is of a kind that
permits the formation of many compounds, called lamellar or
intercalation compounds, by penetration of molecules or ions [37,
38]. Graphitic oxide and graphitic fluoride are nonconducting lamellar
substances not obtained in true molecular forms that can be reproduced,
but their formulas do approximate, respectively, the compositions of
carbon dioxide and carbon monofluoride.
The present study is focused primarily upon the electrical and
mechanical properties of catalytically-grown multi-walled carbon
nanotube-reinforced epoxy composite materials. Development of carbon
nanotube-based, and particularly multi-walled carbon nanotube-based,
polymer nanocomposites is attractive because of the possibility of
combining the extraordinary properties of carbon nanotubes with the
lightweight character of polymers to develop unique, tailorable
materials. On the basis of the extraordinary mechanical properties and
the large aspect ratio associated with individual tubes, carbon
nanotubes are excellent candidates for the development of
nano-reinforced polymer composite materials. However, assurance of
homogeneous dispersion, interfacial compatibility between the carbon
nanotube and the polymer, and exfoliation of the aggregates of carbon
nanotubes, are required for the successful integration of carbon
nanotubes into nanocomposites. Carbon nanotubes, according to the
present invention, include, but are not limited to, single-walled carbon
nanotubes, multi-walled carbon nanotubes, double-walled carbon
nanotubes, buckytubes, small-diameter carbon nanotubes, fullerene tubes,
tubular fullerenes, graphite fibrils, carbon nanofibers, and
combinations thereof. Such carbon nanotubes can be of a variety and
range of lengths, diameters, number of tube walls, and chirality, and
can be made by any known technique. The present study aims to provide a
fundamental understanding of the electrical and mechanical properties of
catalytically-grown multi-walled carbon nanotube-reinforced epoxy
composite materials. Particular emphasis is placed upon the effect of
carbon loading on the electrical conductivity and the influence of
temperature on the loss factor and modulus for the composite materials.