Ceramic matrix composites DJM
Ceramic matrix composites (CMCs) have been developed to overcome the intrinsic brittleness and lack or mechanical reliability of monolithic ceramics, which are otherwise attractive for their high stiffness and strength.
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The issue is particularly acute with glasses, as the amorphous structure does not provide any obstacle to crack propagation and the fracture toughness is very low (<1MPa.m1/2).
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In addition to mechanical effects, the reinforcing phase ,may benefit other properties such as electrical conductivity, thermal expansion coefficient, hardness and thermal shock resistance.
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The combination of these characteristics with intrinsic advantages of ceramic materials such as high-temperature stability, high corrosion resistance, light weight and electrical insulation, makes CMCs very attractive functional and structural materials for a variety of pplications; they have particular relevance under harsh conditions where other materials (e.g. metallic alloys) cannot be used.
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A wide range of reinforcing fibres have been explored, including those based on SiC, carbon, alumina and mullite.
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Various toughening mechanisms can be involved, including fibre debonding, fibre pull-out, and crack bridging.
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However, carbon fibres are amongst the highest performance toughening elements investigated, since the first reports of their use in ceramic matrices were published in the late 1960s.
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The fracture toughness of carbon and SiC fibre reinforced glass and ceramic matrix composites can be much better than the native matrix, as demonstrated by a wealth of available data in the literature (e.g. 17MPa.m1/2 for SiC fibre reinforced glass-ceramic composites).
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Although their remarkable properties have suggested applications as diverse as tissue scaffolds, field emission guns, and supercapacitor electrodes, the interest in composite materials is driven by both the mechanical and functional properties that can be obtained at very low density (typically in the range 1.5-2.0 gcm-3 ). For individual perfect CNTs, the axial stiffness has been shown to match that of the best carbon fibres (approaching around 1 TPa), whilst the strength is an order of magnitude higher (around 50 GPa).
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Their electronic properties depend subtly on the exact structure but larger CNTs are essentially metallic conductors, smaller CNTs can offer unique optoelectronic properties, useful, for example, in non-linear optics.
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Ballistic electron transport effects can be related to uniquely high current carrying capacity (up to 109 Acm-2 ) whilst the axial thermal conductivity is higher than that of diamond (>2000 Wm-1K-1).
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It is worth noting that surface areas of CNTs can be very high since, in the absence of agglomeration, with every atom of a single walled
nanotube lies on its surface; however, this factor can be a mixed lessing when considering composite applications, as discussed further below.
One other significant characteristic of CNTs is their very high aspect (length to diameter) ratio which is relevant to load transfer with the matrix and, hence, effective reinforcement. Standard continuous-fibre composites have excellent anisotropic structural properties combined with low density, but are expensive to process and are limited to simple shapes.
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Short-fibre composites, on the other hand, are easier to produce in complex shapes but with conventional fibres, the aspect ratio is typically limited to around 100, after processing.
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In principle, the small absolute length of CNTs, combined with their resilience in bending, allows them to be manipulated with conventional processing equipment, potentially retaining their high aspect ratio; however, in practice, length degradation is known to occur under high shear conditions. The high aspect ratio of CNTs can also encourage the formation of percolating networks that are relevant to functional properties, particularly electrical conductivity; indeed the lowest percolation threshold for any system has been observed in kinetically-formed networks of CNTs in epoxy.
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Structurally, CNTs have diameters in the range of around 1 nm to a somewhat arbitrary upper limit of 50 nm, and lengths of many microns (even centimetres in special cases)
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They can consist of one or more concentric graphitic cylinders, forming single or multi walled nanotubes (SWCNTs / MWCNTs). In contrast, commercial (PAN and pitch) carbon fibres are typically in the 7 – 20 µm diameter range, whilst vapour-grown carbon fibres (VGCFs) have a broad range of intermediate diameters. Compared to carbon fibres, the best nanotubes can have almost atomistically perfect structures; indeed, there is a general question as to whether the smallest CNTs should be regarded as very small fibres or heavy molecules, especially as the diameters of the smallest nanotubes are similar to those of common polymer molecules.
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Consequently, it is not yet clear to what extent conventional fibre composite understanding can be extended to CNT composites, or whether new mechanisms will emerge.
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Although the perfect CNT structure is very appealing, real materials are very diverse and vary significantly in terms of dimensions, purity, surface chemistry, crystallinity, graphitic orientation, degree of entanglement, and cost. These factors directly affect the properties and processability of CNTs and they must be considered when interpreting their performance in a given application. In very broad terms, CNTs can be divided into two classes depending on the synthetic route used to prepare them.
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