Abstract
The main objects of this thesis include: (a) preparation of carbon tubes via a novel chemical approach, (b) investigation of their formation mechanism, (c) studying how the graphitization treatments affect their properties, and (d) application of these carbon tubes for electron field emitting. Partially ordered micrometer-sized carbon tubes of several centimeters in length have been prepared via a new chemical approach by the pyrolysis of composite fibers consisting of a thermally more stable polypyrrole (PPy) skin layer and a PET core. The wall thickness of resultant hollow carbon tubes was found to be directly proportional to the thickness of the original PPy coating. Elemental analysis results indicated that these tubes essentially consisted of pure carbon, accompanied with small amount of N (< 3 wt %) and H (< 1 wt %). The use of single-pulse magic-angle-spinning solid-state 13C NMR showed that the carbon of these tubes was unsaturated in nature. The XRD diffraction pattern for the ground-up carbon tubes formed at 1000 °C showed two diffraction bands with the maxima at 2θ= 25.86 and 43.8°, which is equivalent to a d-spacing of 3.45 and 2.07 Å, respectively. The Raman spectrum for the same tubes showed two bands at ca. 1354 and 1584 cm-1. This new method enables the control on both the diameter and wall thickness of the carbon tubes. Most interestingly, it also provides a feasible method for the preparation of two or three dimensional well-organized carbon tube assembly from suitable woven fabrics or structures. The formation mechanisms of the PPy/PET composite fibers and their corresponding carbon tubes have also been studied with the aids of various spectroscopic and analytical methods. The investigations indicated that the morphological quality, integrity, and thickness of the PPy coating layer are the most crucial properties in determining the success of obtaining hollow and opened carbon tube structures. Observations with SEM on the pyrolyzed samples indicated that the PET core of a composite fiber started to melt at between 230-290 ℃, then decomposed and almost disappeared at ca. 390 ℃, leaving behind only the hollow sheath. Both the diameter and wall-thickness of such hollow tube further decreased as the treatment temperature elevated during the subsequent carbonization stage. The released gaseous, as well as the sublimed solid by-products during the carbon tube formation process were also monitored and investigated. All results of the present study indicate that PPy is thermally more stable than PET, thus suggest that the carbon tube walls were mainly derived from the PPy skin layers. Moreover, to validate the general strategy employed in this study, other type of thermally stable conducting polymer (such as polyaniline) and other type of thermally removable fibers (such PP, Nylon, silk, cotton and wood cotton) has also being used as the respective skin layer and core fiber materials. These resultant carbon tubes were found to be amorphous carbon. After further annealing at 1000-2400 ℃, the carbon tube structure was found, based on XRD, Raman, and TEM studies, to change gradually from a disordered amorphous phase to a highly ordered graphitic phase with preferred orientation. As the annealing temperature from 1000 to 2400 ℃, the graphitic crystallites of carbon tubes not only increased their sizes considerably but also tilted their stacking planes gradually toward the tube axis. Both SEM and SAD results implied that the 2400 ℃ annealed sample may have a cylindrical layer-stacking structure similar to those of carbon nanotubes. Accompanying these enhancements of structural ordering and orientational preference of the graphitic planes, the conductivity along the tube axis and the oxidative thermal stability of the corresponding carbon tubes was also found to increase significantly. Furthermore, the oxidative thermal stability results (obtained with TG-DTA) of these thermally annealed carbon tubes were correlated with their microstructures (observed with HRTEM). Interestingly, all the DTG (differential thermogravimetric) traces of these annealed carbon tubes (at between 1600-2400 ℃) can be resolved into three distinct peaks, with the area percent of the highest temperature peak being coincided with the graphitization index as measured by XRD. These three peaks appeared at the reaction temperatures ranging from low to high may be attributed to the contribution from the disordered, turbostratic, and graphitic carbons, respectively. Our new chemical approach can also be utilized to prepare novel carbon-tube arrays that have hundreds or thousands of parallel carbon tubes in a bundle. Such assembled structures are believed to be particularly suitable for the application on electron field emitting displays. These carbon tube arrays, after being electrochemically etched to sharpen the tip of the individual carbon tubes within the bundle, were found to show rather good electron emitting properties, with the turn-on field being as low as 3.8 V/μm.