Electrical and thermal properties of carbon nanotube bulk materials: Experimental studies for the $328–958\mathrm{K}$ temperature range

We report on electrical and thermal properties in the temperature range from $328\text{to}958\mathrm{K}$ of multiwall carbon nanotube (MWNT) bulk materials that were consolidated by spark plasma sintering. The rather dense MWNT bulk materials show exclusively nonmetallic temperature dependence of electrical conductivity from $328\text{to}958\mathrm{K}$, owing to the absence of metallic conduction mechanism in such a highly disordered system. The conductivity exhibited extremely weak temperature dependence with only 35% increase of room-temperature conductivity at $958\mathrm{K}$, which was explained by a heterogeneous model considering both fluctuation-assisted tunneling between nanotubes or shells of MWNT and variable-range hopping between graphite microphases that were observed to be dispersed in MWNT bulk materials. The results suggest that fluctuation-assisted tunneling governed this weak conductivity-temperature dependence. Metallic diffusion behavior was observed from $328\text{to}958\mathrm{K}$, and it indicates that phonon drag contributed little to the thermoelectric power of MWNT bulk materials. By contrast, we further show that the increase in sample dimensionality from individual MWNT to bulk materials tends to increase the metallic temperature dependence of electrical conductivity and remarkably decrease the magnitude of thermal conductivity. The geometric shift from graphene sheet to tubular nanotube for carbon-related bulk materials changes the conduction from the combination of $n$ and $p$ types to absolute $p$ type.

Figure 1

(a) TEM image and (b) HRTEM image of the MWNT starting powders.

Figure 2

(a) TEM image and (b) HRTEM image of the MWNT bulk materials spark plasma sintered at $1873\mathrm{K}$ for $5\min$.

Figure 3

XRD patterns of (a) the MWNT starting powders and (b) the MWNT bulk materials spark plasma sintered at $1873\mathrm{K}$ for $5\min$. The inset clearly shows that the (101) Bragg peaks were detected in both the powders and bulk materials.

Figure 4

Schematic illustration of graphite microphases embedded between tangled MWNTs. In such a highly disordered system, fluctuation-assisted tunneling is expected between shells of MWNT or through barriers of tube-tube contacts, and variable-range hopping is expected between isolated graphite phases.

Figure 5

(Color online) (a) Electrical conductivity of MWNT bulk materials as a function of temperature. The conductivity data for graphite are given for comparison. (b) Normalized conductivity $\sigma ∕{\sigma }_{\mathrm{RT}}$ of MWNT bulk materials with data fitting to the heterogeneous model in Eq. (3). We also show respectively contributions of two parallel mechanisms of fluctuation-assisted tunneling and variable-range hopping to the conductivity of MWNT bulk materials. (c) Normalized conductivities of MWNT bulk materials, individual MWNT (Ref. 8), and MWNT bundle (Ref. 9) for comparison. The conductivity data for individual MWNT and MWNT bundle are fitted to only fluctuation-assisted tunneling.

Figure 6

(Color online) Thermoelectric power of MWNT bulk materials as a function of temperature. The $S$ data for graphite, individual MWNT (Ref. 12), and individual SWNT (Ref. 11) are shown for comparison. The $S$ values were measured in the temperature range from $328\text{to}958\mathrm{K}$ for MWNT bulk materials, from $4\text{to}320\mathrm{K}$ for individual MWNT (Ref. 12), and from $100\text{to}270\mathrm{K}$ for individual SWNT (Ref. 11). The $S$ data for the three carbon nanotube samples were respectively extrapolated by a linear fitting to low temperatures to obtain the corresponding $S$ values at zero temperature.

Figure 7

(Color online) Plot of $\ln \sigma$ vs $1∕T$ showing temperature dependence of the electrical conductivity. The six data points measured at $T>700\mathrm{K}$ were used for linear fitting of electrical conductivities.

Figure 8

The measured total thermal conductivity of the MWNT bulk materials as a function of temperature. The solid line simply connects data points, and the dashed line represents the calculated lattice thermal conductivity, $k_L$.

Figure 9

Temperature dependence of the thermoelectric figure of merit $Z$ of MWNT bulk materials. The measurement was carried out in a helium atmosphere.