The thermal conductivity of single-wall carbon nanotubes (SWCNTs) is predicted to increase with length, but this has never been proved experimentally because of limitations in previous measurement methods. Here, the authors report the measurement of the length-dependent thermal conductivities of individual SWCNTs on a Si substrate using a four-pad 3ω method. An increase in thermal conductivity with length was observed at room temperature, which is consistent with a theoretical prediction that considers higher order three-phonon processes. When SWCNTs are longer than the phonon mean path, they showed dissipative thermal transport. The observed increase of thermal conductivity with length makes SWCNTs ideal for thermal management.

In this paper, we propose a modified wavevector (WV) model that takes account of the N-process relaxation time and second-order three-phonon process to predict the length dependence of the thermal conductivity of single-wall carbon nanotubes (SWNTs). The model is validated by length-dependent thermal conductivities of individual SWNTs measured using the four-pad 3ω method. The fitted Grüneisen parameter is close to 2 for SWNTs. These results indicate that the effect of the second-order three-phonon process cannot be neglected at room temperature. Both the experimental and theoretical results prove that the thermal conductivity increases with length of SWNTs over the range of 0.5–7 µm.

In recent two decades, the 3ω technique has been proven to be valuable for characterizing thermophysical properties of materials from nanoscale to bulk, but some inherent deficiencies in this technique such as laborious and repeated four-pad micro strip heater/sensor deposition process and flimsiness of the micro heater/sensor limit its practical applications. Here, the authors report a novel 3ω technique, based on a freestanding sensor replacing the conventional 3ω heater/sensor adjacent to the specimen surface. A zigzag temperature response curve of the new sensor instead of the classical straight line was observed and used to extract the specimen thermal conductivity. Experimental results which excellently agree with calculation values show that the new technique is of great application value to thermal properties characterization of amorphous bulks and hundreds of microns thick wafers.

The thermal conductivities, thermal diffusivity, thermal anisotropy ratio, and thermal boundary resistance for the multilayered microstructure of a carbon nanotube (CNT) array are reconstructed experimentally using the 3ω method with two different width metal heaters. The thermal impedance in the frequency domain and sensitivity coefficients are introduced to simultaneously determine the multiple thermal parameters. The thermal conductivity at 295 K is 38 W · m −1 · K −1 along the nanotube growth direction, and two orders of magnitude lower in the direction perpendicular to the tubes with the anisotropy ratio as large as 86. Separation of the contact and CNT array resistances is realized through circuit modeling. The measured thermal boundary resistances of the CNT array/Si substrate and insulating diamond film interfaces are 3.1 m 2 · K · MW −1 and 18.4 m 2 · K · MW −1 , respectively. The measured thermal boundary resistance between the heater and diamond film is 0.085 m 2 · K · MW −1 using a reference sample without a CNT array. The thermal conductivity for a CNT array already exceeds those of phase-changing thermal interface materials used in microelectronics.

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