Hierarchical composites

Nanocarbon composites can be broadly divided into three kinds, each with some possible subdivisions. Examples of these composites and their schematic representations are presented in Fig. 8.1. The first type corresponds to composites where the nanocarbon is used as a filler added to a polymer matrix analogous, for example, to rubber reinforced with carbon black (CB). The second consists of hierarchical composites with both macroscopic fibers and nanocarbon in a polymer, such as a carbon fiber laminate with CNTs dispersed in the epoxy matrix. The third type is macroscopic fibers based... 

Fig. 8.1 Electron micrographs of different nanocarbon composite types (top) and their schematic representation (bottom). The nanocarbons can be dispersed as a filler (left), combined with macroscopic fibers in a hierarchical composite (middle), or assembled as a continuous nanostructured fiber (right). Micrographs from references [7, 8, 9], with kind permission from Elsevier (2010, 2008, 2009).

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... 

Fig. 8.6 Hierarchical composites of nanocarbon, macroscopic fiber and polymer matrix.

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. 

Similarly, by directly measuring changes in electrical resistance during mechanical deformation it is possible to monitor crack propagation in hierarchical composites with non-conductive fibers and CNTs dispersed in the matrix [48]. Figure 8.7(b) shows... 

Fig. 8.7 Examples of hierarchical composites where the presence of CNTs is used for SHM. (a) Damage detection through thermal imaging of resistively-heated CNTs in an alumina composite [47] and (b) detection of crack propagation by monitoring electrical resistance (normalized by specimen length) in a CNT/glass fiber/epoxy composite [48], With kind permission from IOP (2011) and Wiley (2006).

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 mechanical reinforcement, the presence of nanocarbons in these hierarchical composites can also be used for piezoresistive structural health monitoring or damage evaluation by thermal imaging. Other functions of the nanocarbon, for example in structural supercapacitors, are likely to emerge in the near future. 

Qian H, Greenhalgh ES, Shaffer MSP, Bismarck A. Carbon nanotube-based hierarchical composites a review.yMaferChem. 2010 20(23) 4751-62. 

Stahl elaborated a more complex, hierarchical composition of bodies. All natural bodies were either simple or compound. Simple bodies were principles. Compound bodies had three levels of composition mix d, compound, and aggregate. Principles composed mixts directly. Mixts, in turn, composed compounds and aggregates. In other words, one had to make a distinction between the original mixts ( mixts consisting of principles ) and the secondary mixts ( bodies compounded of mixts ). Ideally, principles had to be simple substances that existed in the mixts before chemical analysis and to which mixts were resolved after the analysis, as had long been prescribed in the French didactic tradition ... 

Agglomerates of nanodiamond feature a hierarchical composition that is characterized by very tightly bound primary aggregates and by grape-like secondary... 

The synthesis of a hierarchical pore structure, combining the macroporous diatomaceous earth with microporous zeolites, is reported. Diatomaceous earth is an abundant and varied source of macroporous silica which has been zeolitisatised to produce a bifunctional, hierarchical composite. A range of different zeolites have been synthesised to generate different pore architectures, hydrophobic/hydrophilic materials and ion-exchange/catalytic properties. 

Keywords Bacterial cellulose Hierarchical composites Mechanical properties Natural fibres Surface modification... 

Juntaro J (2009) Envirorunentally friendly hierarchical composites. PhD Thesis, Department of Chemical Engineering. Imperial College, London, 207 p... 

Wood is by far the primary source of cellulose, though it also occurs in plant fibers and in the shells of tunicates (a sea animal), and is also produced by certain bacteria. Wood fibers exhibit a complex, hierarchical composite structure, consist-... 

Gibson RF (2010) A review of recent research on mechanics of multifunctional composite materials and structures. Compos Struct 92 2793-2810. doi 10.1016. compstract.2010.05.003 Qian H, Greenhalgh ES, Shaffer MSP, Bismarck A (2010) Carbon nanotebe-based hierarchical composites a review. J Matm- Chem 20 4751. doi 10.1039/c000041h Svancara I, Walcarius A, Kalcher K, Vytfas K (2009) Carbon paste electrodes in the new millennium. Cent Eur J Chem 7 598-656. doi 10.2478/sl 1532-009-0097-9... 

Vertical compliance. Also called substituability. The goal is to check whether it is possible to replace a set of primitive components that are nested inside a composite component by the composite component itself. In other words, this compliance can answer the question whether the architecture description of the system is sound with respect to the hierarchical composition of the components. 

Lee KY, Ho KKC, Schlufter K, Bismarck A (2012b) Hierarchical composites reinforced with robust short sisal fibre preforms utilising bacterial cellulose as binder. Compos Sci Technol 72 1479-1486... 

Figure 4.5 Bright-field TEM of the hierarchical composite structure (at the pm and nm length scales) of melt-processed PET/organo-MMT nanocomposites, (top) Melt-processed copolymer-PET/3 wt% Ci6H33-imidazolium MMT boxes indicate the region of the subsequent higher-magnification image [44]. (middle) Melt-processed homopolymer-PET/3 wt% CieHaa-imidazolium MMT [44]. (bottom) Melt-processed homopolymer-PET/3 wt% Ci6H33-quinolinium MMT [25]. 2010, 2006 Wiley, reproduced with permission.

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