Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

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sintered at 1500 C are responsible for its poorest fracture toughness and lowest bending stress. Reducing the sinter temperature to 1450 C leads to a marked increase of the bending stress (554 MPa) and VIF toughness (3.9 MPa m 1/2 ). The fracture toughness of nanocomposite increases with decreasing sinter temperature. Comparing the fracture toughness values of monolithic alumina sintered at the same temperature, it appears that the toughening efficiency of MWNT in alumina is very low. The fracture toughness of nanocomposite hot pressed at 1500 Cis even smaller than that of alumina sintered at 1500 C. There is no improvement in fracture toughness of the nanocomposite hot pressed at 1450 and 1400 C. The fracture toughness of nanocomposite hot pressed at 1350 C improves by nearly 33.3% compared with the single-phase alumina sintered at 1350 C. The poor toughness of the nanocomposite hot pressed at 1400–1500 C is attributed to the degradation of CNTs [Chap. 5, Ref. 53, Chap. 5, Ref. 69]. It is quite obvious that the hot-pressing temperature must be reduced to lower temperatures to preserve the structure of CNTs. In another study, Sun et al. employed SPS to consolidate the Al2O3/0.1 wt% CNT nanocomposite and single-phase alumina at 1300 C for 5 min [Chap. 5, Ref. 61]. Obviously, the addition of only 0.1 wt% CNT to alumina increases the fracture toughness by about 32.4% from 3.7 to 4.9 MPa m 1/2 . Compared with hot pressing, only one-tenth of CNT content is needed to improve the toughness of alumina by 33%. Figure 7.6(a) and (b) show SEM fractographs of Al2O3/0.1 wt% CNT nanocomposite. It is interesting to see from Figure 7.6(a) that the nanotubes are covered with matrix material, forming bridges across the crack. The crack bridging effect hinders the enlargement of the crack. Other alumina-sheathed nanotubes are broken and embedded in the matrix along the crack (Figure 7.4(b)). The crack bridging mechanism is responsible for the improvement of fracture toughness of such nanocomposite. This behavior is commonly observed in the polymer-CNT nanocomposites toughened by CNTs [20]. The polymer sheathed nanotubes bridge the matrix cracks effectively during tensile loading (Figure 7.7). Figure 7.6 SEM fractographs showing typical interactions between crack and alumina sheathed CNTs. Reproduced with permission from [Chap. 5, Ref. 61]. Copyright Ó (2002) The American Chemical Society. 7.3 Oxide-Based Nanocompositesj193
194j 7 Mechanical Properties of Carbon Nanotube–Ceramic Nanocomposites Figure 7.7 SEM micrograph showing crack bridging by polymer sheathed CNTs. Some nanotubes are sheathed by small polymer beads as indicated by the arrows. Reproduced with permission from [20]. Copyright Ó (2004) The American Chemical Society. Yamamoto et al. fabricated the Al2O3/0.9 vol% MWNT, Al2O3/1.9 vol% MWNT and Al2O3/3.7 vol% MWNT nanocomposites by dispersing acid-treated MWNTs in ethanol ultrasonically. Aluminum hydroxide and magnesium hydroxide were then added to the nanotube suspension under sonication. The resulting suspension was filtered, oven dried, followed by SPS at 1500 C under a pressure of 20 MPa for 10 min [Chap. 5, Ref. 71]. From the zeta potential measurements, aluminum hydroxide is positively charged over a wide pH range of 3–9 whilst acid-treated MWNTs is negatively changed in this pH range. Aluminum hydroxide particles are accumulated on MWNTs surfaces due to the electrostatic attraction, resulting in a homogeneous dispersion of nanotubes. Aluminum hydroxide precursor finally converted to a-alumina during SPS. The VIF test revealed that the addition of 0.9 vol% MWNT to alumina leads to an increase of fracture toughness from about 4.25 to 5.90 MPa m 1/2 . The toughness then decreases with increasing nanotube content. Figure 7.8 shows the variation of normalized fracture toughness vs nanotube content for the Al2O3/MWNTnanocomposites. In this figure, the fracture toughness value of nanocomposites is normalized to that of pure alumina. The normalized experimental result reported by Fan et al. [Chap. 5, Ref. 54] is also shown for comparison. Apparently, additions of CNTs up to 1 vol% CNTs can produce a beneficial toughening effect in alumina despite the employment of high sintering temperature (1500 or 1600 C). Above 1 vol% CNT, the toughening effect of CNTs diminishes as expected. Mukerjee and coworkers fabricated pure Al 2O 3,Al 2O 3/5.7 vol% SWNTand Al 2O 3/ 10 vol% SWNT nanocomposites through powder mixing in ethanol, ball milling and SPS at 1150 C for 3 min under 63 MPa [Chap. 5, Ref. 44, Chap. 5, Ref. 70]. The fracture toughness and hardness of these specimens were determined by indentation measurements. The indentation fracture toughness of pure Al2O3, Al2O3/5.7 vol% SWNT and Al2O3/10 vol% SWNT nanocomposites is 3.3, 7.9 and 9.7 MPa m 1/2 , respectively. All these specimens achieved a full density of 100% by sintering at 1150 C for 3 min (Chap. 5, Table 5.2). The structure and unique properties of SWNTs can be preserved at 1150 C for 3 min. Under these processing
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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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100j 3 Physical Properties of Carbo
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102j 3 Physical Properties of Carbo
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104j 4 Mechanical Characteristics o
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132j 5 Carbon Nanotube-Ceramic Nano
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