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.