A new structure for multi-walled carbon nanotubes reinforced alumina nanocomposite with high strength and toughness
Introduction
Since the discovery of carbon nanotubes (CNTs), they have been considered as the most promising reinforcements for composite materials due to high aspect ratios and exceptional mechanical characteristics [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Until now, however, most results for toughening have been disappointing, only very little or no improvements of toughening were reported in carbon nanotube reinforced ceramic. For instance, 10 vol.% MWCNT/SiC composite showed only a 10% increase in bending strength and toughness over monolithic SiC [3]. Siegel et al. [9] reported that 10 vol.% MWCNT/Al2O3 nanocomposites showed a 24% increase of fracture toughness over pure alumina. Although Peigney [6], [7], [8] used an in situ vapour method to prepare CNTs/Fe/Al2O3 composites with CNTs uniformly dispersion in the matrix, mechanical properties showed no obvious improvement due to its low CNTs purity and damage during sintering.
Zhan et al., reported significant fracture toughness improvements, three times higher than an unreinforced matrix, based on indentation measurements [1]. However, Sheldon et al. [14] pointed out that the toughness of the nanocomposite may be severely overestimated when measured by indentation method. At the same time, Wang et al., fabricated CNT-alumina materials using the method of Zhan et al., and performed the far more reliable single edge V-notched beam test, which showed no enhanced toughening and thus refuted the claims of high toughness by Zhan et al.[2]. It is considered crack deflection and crack-bridging mechanisms are ineffective in matrix due to small toughening-zone sizes resulting from the fine length of SWNTs comparing with size of alumina powders [15], [16], [17]. New work by Xia et al.[5] has prepared a thin (20–90 μm thickness) alumina composites reinforced with a highly ordered and aligned MWCNTs. It has been directly observed collapse of the MWCNTs, crack deflection, crack bridging, and MWCNTs pullout induced by nanoindentation.
In this study, the massive CNT-Al2O3 composites with a new CNTs dispersion structure were prepared by hot-pressing. The toughening mechanisms were directly observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), and the relationships between CNTs microstructure and mechanical properties of the composites were also discussed.
Section snippets
Experimental
Alumina powder with particle sizes of 70 nm was obtained from Jiangsu nanotech Corporation (China). MWCNTs samples used in this work were prepared by the catalytic decomposition of propylene on Fe/Al2O3 catalyst [18], [19]. To disperse nanotubes homogeneously in the alumina, three steps were used. First, CNTs were treated by a mixed acid (98% sulfuric and 68% nitric acids, 3:1) in ultrasonic bath in order to remove impurity and open some agglomerates of CNTs. The resulting CNTs and sodium
Results and discussion
Fig. 1 shows the configuration of the as-grown CNTs. It can be observed that the CNTs are made up of many agglomerates (8–20 μm) due to preparation in a Nano-Agglomerate Fluidized-Bed Reactor (NAFBR) [18], [19], as shown in Fig. 1a. The eventual structures of CNTs by TEM observation (Fig. 1b) show that agglomerates (100–200 nm) are randomly entangled and cross-linked, and the length of CNT is 1–2 μm.
The densities of 3 vol.% CNTs/Al2O3 (3.90 g cm− 3) is almost the same as that of pure alumina
Conclusions
MWCNTs/Al2O3 composites with a new CNTs dispersion structure have been prepared by hot-pressing. A fracture toughness increase of 79% and strength increase of 13%, comparing with that of pure nanocrystalline alumina, can be achieved. It is suggested that CNTs form a pin-like network in the matrix, some CNTs bundles siting along the Al2O3 grains and others embedding into the grains, which contribute to toughness increase and strength increase respectively. Multi-wall carbon nanotubes are
Acknowledgements
This research was partially supported by the Foundation of National Nature Science (20236020), Harbin Foundation of Innovation Plan for Young Scientists, China (2006RFQXG030) and the Science Foundation for Post Doctorate Research from Hei Longjiang Province, China.
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