Nanocarbons properties

In this section, different nanocarbons and their chemical and physical properties are discussed (for more details see Chapters 1 and 2). Furthermore, the types of defects that can be embedded within these carbon nanostructures are explained, as well as their resulting chemical and physical properties. 

Nanocarbon structures such as fullerenes, carbon nanotubes and graphene, are characterized by their weak interphase interaction with host matrices (polymer, ceramic, metals) when fabricating composites [99,100]. In addition to their characteristic high surface area and high chemical inertness, this fact turns these carbon nanostructures into materials that are very difficult to disperse in a given matrix. However, uniform dispersion and improved nanotube/matrix interactions are necessary to increase the mechanical, physical and chemical properties as well as biocompatibility of the composites [101,102]. 

The greatest advantage of in situ methods over ex situ processes is the benefit of using the nanocarbon as a substrate, template and heat sink for stabilizing metastable phases and small particle sizes and creating hybrids with unusual morphologies [232]. This enables the synthesis of new hybrid materials that may offer new properties and unknown potential for future research and application. 

As subsequent chapters will document, the type, structure and quality of the nanocarbon have a considerable impact on the final performance of the nanocarbon hybrid. Currently, most publications on the synthesis of nanocarbon hybrids focus on GO, which is both easy to prepare and simple to hybridize. However, the mechanical and electrical properties of GO (and also RGO) are often inferior to their pristine counterparts and in fact closer to those of activated carbon. Hence, we recommend always synthesizing and comparing various types of nanocarbons with different features and functionalizations. 

Fig. 6.6 (a) A comparison of mechanical and thermal properties of PMMA hybrids with SWNT, expanded graphite (EGr) and functionalized graphene sheets (FGS). (b) Percentage synergy in hardness and modulus of the hybrids with binary nanocarbon fillers (from [44]). 

The filler route has proved to be very efficient to obtain isotropic composites with relatively large improvements in matrix properties at small mass (volume) fractions of nanocarbon. For example, electrical percolation in epoxy has been obtained with only 0.0025 wt% of multi-wall nanotubes (MWNTs) [12]. Similarly, a 2.7-fold increase in matrix modulus has been observed on addition of 0.6 vol% MWNTs to polyvinyl alcohol (PVA) [13]. Although more modest compared to the previous two examples, a... 

In this simple form, this expression is a good first approximation to compare the experimental reinforcement achieved upon addition of filler to the matrix, to the theoretical prediction [11]. It provides a measure of how efficiently the properties of the nanofiller are exploited in the composite, but also enables the comparison with the level of reinforcement achieved using other fillers. Note, in addition, that equation (8.2) sets an upper limit between Efl5 = 200 GPa and / = 1000 GPa, depending on whether the nanocarbon is randomly or perfectly oriented (without taking q0 into account). 

The research on nanocarbons dispersed in polymer matrices in recent years has shown that this route is very efficient at small volume fractions above electrical percolation, where it can be the basis for new composite functionalities in terms of processing and properties. It is also clear that there is an inherent difficulty in dispersing these nanoscopic objects at high volume fractions, which therefore limits composite absolute properties to a very small fraction of those of the filler. Independent of their absolute properties, composites based on dispersed nanocarbons have served as a test ground to understand better the basic interaction between nanocarbons and polymer matrices, often setting the foundation to study more complex composite structures, such as those discussed in the following sections. 

Another approach to exploit the properties of nanocarbons consists in integrating them in standard fiber-reinforced polymer composites (FRPC). The rationale behind this route is to form a hierarchical composite, with the nanocarbon playing a role at the nanoscale and the macroscopic fiber providing mainly mechanical reinforcement. This strategy typically aims to give FRPCs added functionality, improve their interlaminar properties and increase the fiber surface area. The first two properties are critical for the transport industry, for example, where the replacement of structural metallic... 

Tab. 8.2 Property improvement in hierarchical nanocarbon/polymer composites [45].

The incorporation of nanocarbons in hierarchical composites can also result in large improvements in their electrical conductivity, and to a lesser extent in their thermal conductivity. For ceramic fibers both in-plane and out-of-plane electrical conductivities are increased by several orders of magnitude [41], whereas for CF the improvement is significant only perpendicular to the fiber direction due to the already high conductivity of the fiber itself [46]. The out-of-plane electrical conductivity of CNT/CF/epoxy composites is approaching the requirements for lightning strike protection in aerospace composites, thought to be around 1 10 S/m. Yet further improvements are required, as well as the evaluation of other composite properties relevant for this application, such as maximum current density and thermal conductivity. 

These methods are based on the electrical conductivity of a low volume fraction of CNTs in the matrix and are therefore more suitable for non-conductive fibers (glass, polymeric). In hierarchical CF composites, electrical conduction is dominated by the properties of the macroscopic fibers present at much higher volume fraction, which cannot be decoupled from those of the nanocarbons. 

Hierarchical composites produced by the addition of nanocarbons to standard FR-PCs have tremendous potential. First, because the role of the nanocarbon is to produce only moderate improvements in the absolute properties of the material or to give it additional functionality, these effects being potentially attainable with low mass fraction of nanocarbons. Second, because the ethos itself of hierarchical composites means that rather than competing with well-established composites, nanocarbons are integrated into them to improve their performance and extend their application range. 

In addition to their potential use as structural composites, these macroscopic assemblies of nanocarbons have shown promise as mechanical sensors [83], artificial muscles [84], capacitors [85], electrical wires [59], battery elements [85], dye-sensitized solar cells [86], transparent conductors [87], etc. What stands out is not only the wide range of properties of these type of materials but also the possibility of engineering them to produce such diverse structures, ranging from transparent films to woven fibers. This versatility derives from their hierarchical structure consisting of multiple nano building blocks that are assembled from bottom to top. 

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