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 $120.4\mathrm{A}{\mathrm{cm}}^{-2}{\mathrm{K}}^{-2}$. The electronic transport properties were studied in low temperature ranging from $3\text{to}390\mathrm{K}$ and high temperature around $1100–2300\mathrm{K}$. 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 $1800\mathrm{K}$ above which $R$ increases due to the electron-phonon scattering at high $T$.

Figure 1

(Color online) (a) MWCNT sheet drawn from superaligned MWCNT arrays grown on $4\mathrm{in.}$ silicon wafer by LPCVD method. (b) As-fabricated shrunk yarn rolled on a spool with length about tens of meters. (c) and (d) are SEM images of the MWCNT sheet and the shrunk yarn, respectively.

Figure 2

(Color online) Heating power $UI$ versus $\sigma T^4$ of both the MWCNT sheet and the yarn. The top right inset is the hot suspended MWCNT sheet heated by a dc source under $\sim 1\times {10}^{-5}\mathrm{Pa}$ vacuum and the bottom right inset is the hot shrunk yarn.

Figure 3

(Color online) (a) The $\log I_a\sim \sqrt{U}$ curves of the MWCNT sheet. The straight lines represent the results of linearly fitting in accelerating field regime. (b) Experimental data (solid square) and fitted line in $\log (I_0∕T^2)$ versus $1∕T$ plot of the MWCNT sheet. (c) The $\log I_a\sim \sqrt{U}$ curves of the shrunk yarn. (d) Experimental data (solid square) and fitted line in $\log (I_0∕T^2)$ versus $1∕T$ plot of the shrunk yarn.

Figure 4

(Color online) The emission efficiency versus $T$ of both the MWCNT sheet and the shrunk yarn in which the black solid circles are the experiment data for the MWCNT sheet, the red solid squares are the experiment data for the shrunk yarn, and the blue solid line is the theory curve with $\varphi =4.6\mathrm{eV}$ and $A=120.4\mathrm{A}{\mathrm{cm}}^{-2}{\mathrm{K}}^{-2}$. The inset is the magnified plot of the emission efficiency data in the green rectangle, indicating that $\eta$ is identical for the sheet and the shrunk yarn working at the same temperature.

Figure 5

(Color online) $I\text{−}V$ characteristics of the suspended MWCNT sheet and the shrunk yarn. The top left inset is the $I\text{−}V$ curve at low heating power and the bottom right inset is the electric bridge model for the resistance reduction.

Figure 6

(Color online) The in situ observation of the resistance reduction from the MWCNT sheet to the shrunk yarn. The top left inset is the MWCNT sheet covered on a glass substrate with the two ends fixed by silver paste. The bottom right inset is the morphology of the shrunk yarn after the sheet was blown by air.

Figure 7

(Color online) (a) $R$ versus $T$ of the MWCNT sheet and the shrunk yarn measured by four-probe method from $3\text{to}390\mathrm{K}$. $R$ of the shrunk yarn is less than that of the sheet in the full temperature range. (b) $R\left(T\right)$ of the sheet and the shrunk yarn at high temperature and $R$ of the shrunk yarn increasing with increasing $T$ after $T$ is higher than $1800\mathrm{K}$.