Comparative studies of multiwalled carbon nanotube sheets before and after shrinking

The multiwalled carbon nanotube (MWCNT) yarns converted from superaligned MWCNT arrays are self-assembled sheetlike structure in which MWCNTs are aligned along the stretching direction and will shrink into a tight fiber after passing through volatile solvents. In this paper, we report the comparative studies on the physical properties of the MWCNT sheets and shrunk yarns. The major differences between them lie in the surface area and intertube interactions, which result in different mechanical and electrical properties. Their surface areas were measured according to the energy conservation law and Stefan-Boltzmann law, manifesting that the surface area of the sheet is around 30 times larger than that of the shrunk yarn. With the measured surface area, the thermionic emission from them was further studied. There is no difference in their work function, emission efficiency, as well as Richardson's emission constant which is approximately equal to the theoretical value of 120.4 A cm(-2) K-2. The electronic transport properties were studied in low temperature ranging from 3 to 390 K and high temperature around 1100-2300 K. In all the temperature range, there exists a resistance reduction from the MWCNT sheet to the shrunk yarn which was attributed to the increase of intertube crosstalk points between neighboring MWCNTs. It is also found that R decreases with increasing temperature until 1800 K above which R increases due to the electron-phonon scattering at high T.