Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

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has also been reported by other researchers for the micro-nano ceramic composites [25–28]. The switch from intergranular to transgranular failure is a good indicator for the strength and toughness improvements in ceramic nanocomposites. Second-phase nanoparticles enhance grain boundary properties of ceramics by inducing residual stress as a result of thermal expansion mismatch between the matrix and particles. This internal stress causes an advancing crack to propagate in a tortuous manner rather than a catastrophic straight path. Moreover, second-phase nanoparticle additions also lead to the grain refinement of the ceramic matrix [37]. Reducing grain size promotes the mechanical strength of crystalline materials according to the Hall–Petch relation. In some cases, second-phase nanoparticles also yield elongation of matrix grains. The elongated matrix grains with high aspect ratio increase the fracture toughness by dissipating the fracture energy through crack deflection and bridging mechanisms [30, 31]. Table 5.1 lists the properties of typical micro-nano and nano-nano ceramic composites. Carbon nanotubes (CNTs) with high aspect ratio, extraordinary mechanical strength and stiffness, excellent thermal and electrical conductivity are attractive nanofillers which produce high-performance ceramic composites with multifunctional properties. The reinforcing effect of CNTs with high aspect ratio is considered to be analogous to that of continuous or short-fiber-reinforcement. The superior flexibility of CNTs is very effective in improving the fracture toughness of brittle ceramics. This is accomplished by means of crack deflection at the CNT–matrix interface, crack-bridging and CNT pull-out mechanisms. Recently, Huang et al. reported that SWNTs exhibit superplastic deformation with an apparent elongation of 280% at high temperatures [38]. This finding shows the potential application of CNTs Table 5.1 Mechanical properties of typical micro-nano and nano-nano ceramic composites. Investigators Materials 5.2 Importance of Ceramic-Matrix Nanocompositesj135 Bending strength (MPa) Fracture toughness (MPa m 1/2 ) Anya Ref [25] Micro-nano type: Monolithic Al2O3 (3.5 mm) 430 3.2 Al2O3(4.0 mm)/5 vol% SiC (200 nm) 646 4.6 Al2O3(2.9 mm)/10 vol% SiC (200 nm) 560 5.2 Al2O3(2.6 mm)/15 vol% SiC (200 nm) 549 5.5 Gao et al. Ref [26] Monolithic Al2O3 350 3.5 Al2O3(1.8 mm)/5 vol% SiC (70 nm) 1000 4.0 Zhu et al. Ref [30] Monolithic Al2O3 380 3.9 Al2O3/15vol%Si3N4 (200 nm, inter; 80 nm, intra) 820 6.0 Isobe et al. Ref [33] Monolithic Al2O3 (1.95 mm) 450 3.7 Al2O3 (0.91 mm)/2.8 vol% Ni (150 nm, inter; 300 nm, intra) Nano-nano type: 766 4.6 Bhaduri Ref [34] Al2O3 (44 nm)/10 vol% ZrO2(12.7 nm) — 8.38
136j 5 Carbon Nanotube–Ceramic Nanocomposites as reinforcing fillers for CMCs with improved ductility. Thus, such ceramic-CNT nanocomposite could possess superplastic deformability. Peigney et al. investigated the extruding characteristics of metal oxide-CNT nanocomposites at high temperatures. They indicated that superplastic forming of nanocomposites is made easier by adding CNTs [39]. All these attractive and unique properties of CNTs enable materials scientists to create novel strong and tough ceramic nanocomposites. Moreover, the electrical and thermal conductivities of ceramics can be improved markedly by adding nanotubes. The electrical conductivity of alumina-CNTcomposites can reach up to twelve orders of magnitude higher than their monolithic counterpart. Recent study has shown that the thermal conductivity of alumina-CNT nanocomposites exhibits anisotropic behavior [40]. The nanocomposites conduct heat in one direction, along the alignment of the nanotube axial direction, but reflect heat at right angles to the nanotubes. This anisotropic thermal behavior makes alumina-CNT nanocomposites potential materials for application as thermal barrier layers in microelectronic devices, microwave devices, solid fuel cells, chemical sensors, and so on [40]. 5.3 Preparation of Ceramic-CNT Nanocomposites Despite the fact that the CNTs exhibit remarkable mechanical properties, the reinforcing effect of CNTs in ceramics is far below our expectation. The problems arise from inhomogeneous dispersion of CNTs within the ceramic matrix, inadequate densification of the composites and poor wetting behavior between CNTs and the matrix. All these issues are closely related to the fabrication processes for making ceramic-CNTnanocomposites. As recognized, CNTs are hard to disperse in ceramics. They tend to form clusters caused by van der Waals force interactions. Such clustering produces a negative effect on the physical and mechanical properties of the resulting composites. Individual nanotubes within clusters may slide against each other during mechanical deformation, thereby decreasing the load transfer efficiency. Furthermore, toughening of the ceramic matrix is difficult to achieve if the CNTs agglomerate into clusters. Carbon nanotube clusters minimize crack bridging and pull-out effects greatly. Therefore, homogeneous dispersion of CNTs in the ceramic matrix is a prerequisite of achieving the desired mechanical properties. In most cases, ceramic-CNT composites were prepared by conventional powder mixing and sintering techniques such as pressureless sintering, hot pressing and hot isostatic pressing (HIP). The mechanical properties of ceramic-CNTnanocomposites sintered from these techniques show modest improvement or even deterioration. This is because conventional sintering methods require lengthy treatment at high temperatures to densify green compacts. Such a high temperature environment causes oxidation and deterioration in the properties of CNTs. For example, Ma et al. prepared the SiC/10 vol% MWNT nanocomposite by ultrasonically mixing SiC nanoparticles with CNTs and hot pressing at 2000 C. They reported a modest
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Sie Chin Tjong Carbon Nanotube Rein
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Sie Chin Tjong Carbon Nanotube Rein
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Contents Preface IX List of Abbrevi
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5 Carbon Nanotube-Ceramic Nanocompo
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Preface Carbon nanotubes are nanost
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XII List of Abbreviations HIP hot i
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1 Introduction 1.1 Background Compo
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Figure 1.2 Transmission electron mi
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1.3 Synthesis of Carbon Nanotubes 1
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MWNTs can be as high as 70% of the
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Figure 1.7 In situ TEM images recor
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1.3 Synthesis of Carbon Nanotubesj1
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show TEM images of MWNTs synthesize
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Since then, large efforts have been
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1.3.4 Patent Processes Carbon nanot
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1.4 Purification of Carbon Nanotube
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Table 1.3 Patent processes for the
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Figure 1.13 Schematic illustration
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1.5 Mechanical Properties of Carbon
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Figure 1.14 Carbon nanotubes in hig
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Figure 1.15 In situ tensile deforma
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Table 1.7 Theoretical and experimen
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Nomenclature ~a 1 , ~a 2 Unit vecto
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carbon nanotubes. Physical Review L
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70 Jang, I., Uh, H.S., Cho, H.J., L
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108 Shelimov, K.B., Esenaliev, R.O.
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Bonnamy, S., Beguin, F., Burnham, N
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2 Carbon Nanotube-Metal Nanocomposi
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isostatic pressing. In certain case
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This implies the absence of effecti
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coating material, the plasma gun an
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Table 2.4 Changes in the size and v
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Figure 2.6 (a) Low and (b) high mag
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Figure 2.8 SEM micrographs showing
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Figure 2.11 Bright field TEM microg
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Figure 2.13 TEM image of bulk Al/5
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2.5 Magnesium-Based Nanocomposites
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limited improvement in ultimate ten
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Figure 2.18 SEM micrographs of the
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Figure 2.19 (a) Low and (b) high ma
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Figure 2.22 SEM micrographs of (a)
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Figure 2.25 (a) TEM micrograph show
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2.8 Transition Metal-Based Nanocomp
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Figure 2.27 TEM image of electrodep
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Figure 2.29 SEM micrographs of (a)
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Figure 2.31 Schematic representatio
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einforcement content SiCp/Al compos
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Materials Science Forum, 534-536 (P
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186j 7 Mechanical Properties of Car
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Table 8.1 Patent processes for maki
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218j 8 Conclusions development of c
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220j 8 Conclusions Figure 8.2 TEM m
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222j 8 Conclusions increase in Vick
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224j 8 Conclusions References 1 Cha
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226j 8 Conclusions nano-matrix. Scr
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228j Index l laser ablation 7 load
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