Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

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where p is volume fraction of the filler and pc the percolation threshold. The critical exponents s, s 0 and t are assumed to be universal. The conductivity exponent t reflects the dimensionality of the system with values about 1.3 and 2.0 for two and three dimensional random percolation system, respectively [17]. However, the experimental conductivity exponent t value can deviate from the universal value as a result of electron tunneling from conducting fillers above the percolation threshold [18]. According to the literature, the measured percolation threshold in CNT–ceramic composites is very low, typically in the range of 0.64–4.7 vol%, depending on the type of ceramic matrix and processing technology employed [19, 20]. In terms of a.c. properties, the experimental conductivity and dielectric constant display a pronounced frequency-dependent behavior. The frequency dependence of effective conductivity and dielectric constant of a conductor-insulator composite near the percolation threshold can be described by the following universal power law relation [16]: s / u x e / u y 6.3 Percolation Concentrationj173 ð6:5Þ ð6:6Þ where u is frequency. The critical exponents x and y must satisfy the following relation: x þ y ¼ 1: ð6:7Þ Peigney and coworkers investigated the percolation behavior of in situ MgAl2O3/ CNT nanocomposites having 0.2–25 vol% CNT [19]. The nanocomposites were prepared by catalytic chemical vapor deposition followed by hot pressing at 1300 C. Figure 6.2(a) and (b) shows the variation of electrical conductivity with CNT content for the MgAl2O3/CNTnanocomposites. The inset in Figure 6.2(a) represents a log-log plot of the conductivity as a function of p-pc. From least-square analysis, a linear fit can be obtained according to Equation 6.2, yielding an exponent t ¼ 1.73 0.02. and percolation threshold p c ¼ 0.64 0.02 vol% (about 0.31 wt%). At the percolation threshold, the electrical conductivity increases over seven orders of magnitude from 10 8 to 0.4 S m 1 . Comparing with the ultralow value of pc (0.0025 wt%) for MWNT/ epoxy nanocomposites [7], the percolation threshold is two orders of magnitude higher for the MgAl2O3/CNT nanocomposites. This is attributed to the degradation of CNTs during hot-pressing at 1300 C. Very recently, Ahmad et al. studied the electrical conductivity and dielectric behavior of Al2O3/MWNT nanocomposites prepared by SPS at 1350 C [20]. A percolation threshold of 0.79 vol% was found for such nanocomposites. Further, the dielectric constant can reach as high as 5000 in the low frequency region by adding 1.74 vol% MWNT. Shi and Liang [21] also investigated the percolation and dielectric behavior 3Y-TZP (3 mol% yttria-stabilized tetragonal polycrystalline zirconia) filled with various MWNT contents. The 3Y-TZP/MWNT nanocomposites were fabricated by ball milling constituent materials followed by SPS at 1250 C under a pressure of 60 MPa. Figure 6.3 shows the effective d.c. conductivity vs CNT concentration for the 3Y-TZP/MWNT nanocomposites. Using the least square fit, the
174j 6 Physical Properties of Carbon Nanotube–Ceramic Nanocomposites Figure 6.2 Semi-log plots of electrical conductivity as a function ofCNTcontentintherangeof(a)0–11 vol%CNTand(b)0–25 vol% CNT. Reproduced with permission from [19]. Copyright Ó (2004) Elsevier. percolation threshold and conductivity exponent are determined to be 1.7 wt% (4.7 vol%) and 3.30 0.03, respectively. It is noted that the conductivity exponent t is much higher that that predicted by the universal three-dimensional lattice value (t 2). They explained this in terms of the thermal fluctuation-induced tunneling effect [21, 22]. As the surface of conducting nanotube fillers is covered with a thin
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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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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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