E T. Thostenson et al/ Composites Science and Technology 61(2001)1899-1912 Fig. 7. Micrographs showing the straightness of MWCNTs grown via PECVD [191 Fig 8. Micrographs showing control over the nanotube diameter:(a)40-50 nm and(b)200-300 nm aligned carbon nanotubes [19]- and graphitization is accomplished by changing the tube furnace. Fig. 10 is a SEM micrograph of the fur growth time and temperature, respectively, and applica- nace -grown carbon nanotubes showing the same ran- tion of the dC plasma results in tube growth in the dom, curled structure associated with thermal CVD direction of the plasma. The use of an alternating micro-(Shown in Fig 9). The outer diameters of these tubes wave frequency source to excite the plasma results in the range from 10-50 nm. These tangled, spaghetti-like growth of carbon nanotubes that occur directly normal to nanotubes can be produced at a larger quantity and lower the surface of the substrate. Bower et al. [29] showed that cost than PECVD tubes but there is less control over in microwave plasma-enhanced CVD (MPECV length, diameter, and structure alignment of the carbon nanotubes results from the self- bias that is imposed on the surface of the substrate from the microwave plasma. Fig 9a shows the alignment of 4. Characterization of carbon nanotubes carbon nanotubes grown normal to the surface of an optical glass fiber. To gain further insight into the Significant challenges exist in both the micromechanical mechanism for tube alignment, the tubes were grown for characterization of nanotubes and the modeling of the two minutes under the microwave- induced plasma fol- elastic and fracture behavior at the nano-scale Challenges lowed by 70 min with the plasma off. Fig 9b shows the in characterization of nanotubes and their composites results of this experiment. The upper portion of the include (a) complete lack of micromechanical character nanotubes are straight, indicating alignment in the ization techniques for direct property measurement, (b) plasma, and the base shows a random, curled structure tremendous limitations on specimen size, (c)uncertainty associated with thermal CVD. In addition, the growth in data obtained from indirect measurements, and (d) rate under the plasma enhancement was 40 times faster inadequacy in test specimen preparation techniques and than the thermal CVD lack of control in nanotube alignment and distribution In addition to highly aligned arrays of carbon nano- In order better to understand the mechanical proper tubes, large quantities of carbon nanotubes can be pro- ties of carbon nanotubes, a number of investigators have essed by conventional CVd techniques. Unlike attempted to characterize carbon nanotubes directly PECVD, which requires the use of specialized plasma Treacy et al. [34] first investigated the elastic modulus of equipment, tangled carbon nanotubes are grown in nanotu bes by measuring, in theand graphitization is accomplished by changing the growth time and temperature, respectively, and applica￾tion of the DC plasma results in tube growth in the direction of the plasma. The use of an alternating micro￾wave frequency source to excite the plasma results in the growth of carbon nanotubes that occur directly normal to the surface of the substrate. Bower et al. [29] showed that in microwave plasma-enhanced CVD (MPECVD) alignment of the carbon nanotubes results from the self￾bias that is imposed on the surface of the substrate from the microwave plasma. Fig. 9a shows the alignment of carbon nanotubes grown normal to the surface of an optical glass fiber. To gain further insight into the mechanism for tube alignment, the tubes were grown for two minutes under the microwave-induced plasma fol￾lowed by 70 min with the plasma off. Fig. 9b shows the results of this experiment. The upper portion of the nanotubes are straight, indicating alignment in the plasma, and the base shows a random, curled structure associated with thermal CVD. In addition, the growth rate under the plasma enhancement was 40 times faster than the thermal CVD. In addition to highly aligned arrays of carbon nano￾tubes, large quantities of carbon nanotubes can be pro￾cessed by conventional CVD techniques. Unlike PECVD, which requires the use of specialized plasma equipment, tangled carbon nanotubes are grown in a tube furnace. Fig. 10 is a SEM micrograph of the fur￾nace-grown carbon nanotubes showing the same ran￾dom, curled structure associated with thermal CVD (shown in Fig. 9). The outer diameters of these tubes range from 10–50 nm. These tangled, spaghetti-like nanotubes can be produced at a larger quantity and lower cost than PECVD tubes, but there is less control over length, diameter, and structure. 4. Characterization of carbon nanotubes Significant challenges exist in both the micromechanical characterization of nanotubes and the modeling of the elastic and fracture behavior at the nano-scale. Challenges in characterization of nanotubes and their composites include (a) complete lack of micromechanical character￾ization techniques for direct property measurement, (b) tremendous limitations on specimen size, (c) uncertainty in data obtained from indirect measurements, and (d) inadequacy in test specimen preparation techniques and lack of control in nanotube alignment and distribution. In order better to understand the mechanical proper￾ties of carbon nanotubes, a number of investigators have attempted to characterize carbon nanotubes directly. Treacy et al. [34] first investigated the elastic modulus of isolated multi-walled nanotubes by measuring, in the Fig. 7. Micrographs showing the straightness of MWCNTs grown via PECVD [19]. Fig. 8. Micrographs showing control over the nanotube diameter: (a) 40–50 nm and (b) 200–300 nm aligned carbon nanotubes [19]. E.T. Thostenson et al. / Composites Science and Technology 61 (2001) 1899–1912 1903