Structure and Multiscale Mechanics of Carbon Nanomaterials

High performance carbon fibres are mainly used as reinforcement in fibre-reinforced structural components in aerospace-, automotive-, sports-, and energy applications. For example, many of the lightweight and stiff structural parts of bicycles, sport cars or wind turbine blades are nowadays made of carbon fibre reinforced plastics (CFRP), and the demand for such materials is continuously increasing. The dominant part (more than 90%) of carbon fibres are produced from polyacrylonitrile (PAN) precursor fibres with intermediate moduli of a few 100 GPa, but very high tensile strength up to 8 GPa. The second important class are fibres produced from mesophase pitches (MPP), leading to fibres with extremely high moduli (almost 1000 GPa) as well as good thermal and electrical conductivity. Together with their low weight, chemical resistance, biocompatibility, temperature tolerance and low thermal expansion, carbon fibres may only be beaten by other carbons such as carbon nanotubes (chapter 3) or graphene (chapter 4) as reinforcing materials. Although being much cheaper than those “modern” carbon nanomaterials, still carbon fibres are relatively expensive as compared to, e.g., glass fibres. Nonetheless, the world-wide carbon fibre production is steadily increasing and is expected to double from 68.000 tons in 2015 to 130.000 tons in 2020 (Holmes 2013). This demonstrates that carbon fibres are - and will further remain - the absolutely dominating carbon nanomaterials for light weight structural parts.

In the quest to understand reinforcement by high performance fibres, such as carbon fibres, the development of the subject of composite micromechanics is traced from its earliest roots. It is shown first how, employing concepts introduced by Kelly, it is possible through the use of shear-lag theory to predict the distribution of stress and strain in a single discontinuous fibre in a low-modulus matrix. For a number of years the shear-lag approach could only be used theoretically as there were no techniques available to monitor the stresses within a fibre in a resin. It is then shown that the advent of Raman spectroscopy and the discovery of stress-induced Raman bands shifts in reinforcing fibres, has enabled us to map out the stresses in individual fibres in a transparent resin matrix, and thereby both test and develop Kelly’s pioneering analytical approach.

The pioneering work upon the two-dimensional graphene, a one-atom thick planar sheet of sp²-bonded carbon atoms, was awarded the Nobel Prize in Physics in 2010. Carbon nanotubes (CNTs) are related nanostructures that can be envisaged as being made by rolling the two-dimensional graphene sheets into cylinders. This gives rise to fascinating materials, which have been attracting great deal of research interest in the last two decades, due to their impressive properties and wide range of potential applications. Their applications in mechanical reinforcement and electronic device are particularly promising. The excellent mechanical properties of nanotubes are related to the strong sp² hybridized carbon-carbon bonds and the perfect hexagonal structure in the graphene sheet from which they are built up, while the unique electronic properties are due largely to the one-dimensional confinement of electronic and phonon states which results in van Hove singularities in the density of states (DOS) of nanotubes (Dresselhaus et al., 2005).

Raman spectroscopy has become an important technique to both characterise the electronic structure and follow the deformation behaviour of CNTs. This technique provides insight into their intrinsic properties and the interaction of nanotubes with the surrounding environment, as well as the mechanical reinforcing efficiency of nanotubes in composites.

This chapter aims to give a brief introduction to the structure, preparation and properties of carbon nanotubes, and to review the background and main properties of nanotube Raman bands, with an emphasis on the effect of deformation upon the Raman bands. More comprehensive reviews on the physical properties and Raman spectroscopy of CNTs can be found elsewhere (Dresselhaus et al., 2002 and Dresselhaus et al., 2005).

The study of graphene is one of the most exciting topics in materials science and condensed matter physics (Geim and Novoselov, 2007) and graphene has good prospects for applications in a number of different fields (Novoselov, 2011; Geim, 2011). There has been a rapid rise of interest in the study of the structure and properties of graphene following the first report in 2004 of the preparation and isolation of single graphene layers in Manchester (Novoselov et al, 2004). It had previously been thought that the isolation of single-layer graphene would not be possible since such 2D crystals would be unstable thermodynamically and/or might roll up into scrolls if prepared as single atomic layers (Young et al, 2012). A large number of studies since 2004 have shown that this is certainly not the case. There was excitement about graphene initially because of its electronic properties, with its charge carriers exhibiting very high intrinsic mobility, having zero effective mass and being able to travel distances of microns at room temperature without being scattered (Geim and Novoselov, 2007). Thus the majority of the original research upon graphene had concentrated upon electronic properties, aimed at applications such as using graphene in electronic devices (Avouris, 2010).

Carbon nanotubes (CNTs) have extraordinary mechanical properties due to the stiff sp ² bond resulting in the exceptionally high Young’s moduli in the tera-pascal range, together with their tube structure (Treacy et al., 1996). They have unique electronic properties; they can be either metallic or semiconducting depending on the chirality — the direction along which a tube is rolled up (Odom et al., 1998). Pressure modifies these properties. The sp ² bond stiffens further, and the band gap in semiconducting CNTs changes with pressure (Yang and Han, 2000). To characterize and understand the behaviour of CNTs under pressure, the shift rates of the phonon frequencies with pressure are very interesting, as they directly reflect the mechanics and are closely related to the electronic properties. They can also be used as strain sensors.

In this chapter, we will focus on the shift with pressure of the graphite mode (GM) and the radial breathing mode (RBM). The GM is an in-plane vibrational mode, coming from graphite and characteristic of sp ²-hybridized carbon (Tuinstra and Koenig, 1970). The study of the GM pressure coefficients of CNTs thus provides a direct approach to understand the sp ² bond. It links closely to the high pressure study of other sp ²-bonded materials such as graphene and fullerene. The RBM, though related to the GM, is a unique signature of CNTs (Rao et al., 1997). Its vibrational frequency is diameter-dependent and therefore of critical importance to the study of features, which are related to the tube structure, including the GM pressure coefficients. We will briefly mention other modes, such as the 2D-mode, the second order D-mode from defects, which reflects the change in the electron bands, essential to characterizing graphene (Ferrari et al., 2006).

Nanometer sized particles formed by carbon atoms mainly arranged in a hexagonal atomic structure are called carbon nanostructures (CNS). In this chapter we focus exclusively on sp ²-bonded CNS that include graphene (Geim, 2009; Geim and Novoselov, 2007), single- and multi-walled carbon nanotubes (Iijima, 1991; Pantano et al., 2004), fullerenes (Kroto et al., 1985), and carbon onions (Banhart and Ajayan, 1996; Kroto, 1992; Ugarte, 1992, 1995). Especially graphene has drawn a lot of attention within the last years, because it possesses exceptional mechanical and electrical properties (Geim, 2009; Novoselov et al., 2004) and a high thermal conductivity (Lau et al., 2012). It is the main building block of all other CNS based on sp ²-bonded carbon, which therefore should inherit its exceptional properties making them promising candidates for applications in the field of structural mechanics and the electronics industry, as fillers in nanocomposites (Choi and Lee, 2012; Baughman et al., 2002; Stankovich et al., 2006) and as solid lubricants (Hirata et al., 2004). This chapter will focus on the amazing mechanical properties of CNS only. Information regarding the extraordinary electronic and thermal properties can be found elsewhere (Novoselov et al., 2004; Castro Neto et al., 2009; Balandin, 2011).

In this chapter we discuss the mechanical behaviour of vertically aligned carbon nanotubes (VACNTs) also known as carbon nanotube (CNT) arrays, bundles, brushes, foams, forests, mats, and turfs. VACNTs are complex, hierarchical structures of intertwined tubes arrayed in a nominally vertical alignment due to their perpendicular growth from a stiff substrate. They are a unique class of materials having many of the desirable thermal, electrical, and mechanical properties of individual carbon nanotubes, while exhibiting these properties through the collective interaction of thousands of tubes on a macroscopic scale.