Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

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5.4 Oxide-Based Nanocompositesj145 structure of SWNT of the nanocomposite sintered at 1150 C is still preserved as evidenced by the presence of G-peak at 1595 cm 1 with a shoulder. The shoulder disappears at higher sintering temperatures of 1350 and 1550 C, indicating the loss of CNT structure. Moreover, the peak intensity of D-band relative to the G-band decreases with increasing sintering temperatures, implying an increase in the graphite content. In other words, SWNTs convert to graphite structure at temperatures above 1150 C. The relative density values of the nanocomposite sintered at 1150, 1350 and 1550 C are 100, 98.9 and 97.8%. Apparently, the best combination of microstructure and mechanical properties in alumina-CNT nanocomposites can be attained through heterocoagulation and SPS at lower temperature [60]. Poyato et al. indicated that sintering the colloidally dispersed composite powder at high temperature (1550 C) leads to the conversion of CNTs into disordered graphite, diamond and carbon nano-onions [53]. The disintegration of CNTs at high temperatures degrades the mechanical properties of resulting nanocomposites considerably. Up till now, some workers have been quite unaware of the damage caused by high temperature ( 1250 C) exposure to CNTs. They have still conducted SPS of alumina-CNT nanocomposites at 1500 C [71–73]. The colloidally formed composite powders, as mentioned above, contain organic surfactant or dispersant [60, 61]. More recently, Estili and Kawasaki employed the heterocoagulation technique to prepare the alumina/MWNT nanocomposites without using any organic dispersants for CNTs and alumina powders [74]. Instead, they dispersed acid purified MWNTs and a-alumina powder (150 nm) in water independently. Carboxylic and hydroxyl functional groups formed on acid-treated MWNTs allow them to be dispersed easily in polar water solvent. A CNTsuspension was added to the alumina suspension under a controlled pH of 4.4 (Figure 5.12(a)). The CNT surfaces became coated with alumina particles due to the electrostatic attraction between positively charged alumina particles and negatively charged CNTs at pH 4.4 (Figure 5.12(b) and (c)). Hong and coworkers demonstrated that a molecular level mixing technique can be used to disperse MWNTs homogeneously in an alumina matrix [72]. Acidfunctionalized MWNTs were ultrasonically dispersed in distilled water. Al (NO 3)3 9H2O was added to a CNT suspension followed by sonication and vaporization. The retained powders were oxidized at 350 C for 6 h in air atmosphere, and finally consolidated by SPS at 1500 C for 5 min. The essence of this process is that MWNTs and metal ions are mixed homogeneously in an aqueous solution at the molecular level. Chemical bonding between MWNT and amorphous Al2O3 was developed during the calcination process (Figure 5.13(a) and (b)). However, homogeneous dispersion of nanotubes is only found at low nanotube loading of 1 vol%. Agglomeration of CNTs occurs in the composite at very low filler loading of 1.8 vol%. As mentioned before, the nanocomposites can be classified into intra, inter, intra/ inter or nano-nano depending on the dispersion of second-phase reinforcement in materials [23]. Recent advances in nanotechnology have made possible the design and development of ceramic nanocomposites reinforced with hybrid nanofillers. Very recently, Ahmad and Pan developed novel hybrid alumina nanocomposites
146j 5 Carbon Nanotube–Ceramic Nanocomposites Figure 5.12 (a) Zeta potential values of acid treated MWNTs and alumina at different pH. TEM micrographs showing (b) partial and (c) complete coverage of nanotubes with alumina particles. Reproduced with permission from [74]. Copyright Ó (2008) Elsevier. reinforced with two materials, that is, MWNTs (5, 7 and 10 vol%) and SiC nanoparticles (1 vol%) [75]. The fabrication process consisted of blending MWNTs, SiC and alumina in ethanol ultrasonically followed by ball-milling and drying. Dried composite powders were spark plasma sintered at 1550 C under a pressure of 50 MPa. For these hybrids, SiC nanoparticles are designed to disperse within the grains and grain boundaries of alumina (intra/inter type), and MWNTs are located at grain boundaries as intergranular reinforcements (Figure 5.14). Therefore, hybrid
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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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composite. Materials Science and En
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111 Cha, S.I., Kim, K.T., Arshad, S
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90j 3 Physical Properties of Carbon
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92j 3 Physical Properties of Carbon
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Page 201 and 202: 186j 7 Mechanical Properties of Car
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196j 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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