What is it about?

Carbon nanotubes (CNTs) have been extensively studied in recent decades due to their unique thermophysical properties. They are promising nanomaterials for a variety of applications, including nanoelectronics, energy, medicine and as composite materials, owing to their exceptional electrical, thermal, and mechanical properties. Investigations of CNTs’ heat capacity at low temperatures reveal valuable information about quantum effects, phonon density of states and thermal properties of carbon-based nanomaterials.

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Why is it important?

Low-temperature specific heat of multi-walled carbon nanotubes (MWCNTs) with different grinding was studied. Two sets of modified, milled and oxidized/milled MWCNTs with an average outer diameter of 9.4 nm were used. The experimental results were compared with literature data for different carbon systems: bundles of single-walled carbon nanotubes (SWCNTs), graphite and other MWCNTs. The contributions of phonon spectrum characteristics and intertube interactions were found to be significant factors influencing the heat capacity both in the case of MWCNTs and bundles of SWCNTs. The grinding effect, associated with the reduction of the size of MWCNTs agglomerates, leads to an increased heat capacity. It was demonstrated that the lowest-temperature heat capacity consists of two main contributions: the Debye (C3T3) and the dispersive (C5T5) one. The obtained negative C5 parameter indicated flexural dispersion for phonons. The magnitudes of Debye and flexural dispersive components depend on structural parameters of nanotubes: such as the diameter of individual nanotubes, the average diameter of the bundle and the size of agglomerates. A monotonic proportional correlation was observed between C3 and |C5| parameters: |C5| increases following a power law with an exponent of 1.5 with the increase of C3. The maximum values of C3 and |C5| correspond to SWCNT systems, while the minimum values correspond to MWCNTs. These results show that the heat capacity of the nanotube system increases if the interaction forces between neighbouring SWCNTs in the bundle or between the walls inside of MWCNTs decrease. This dependence is confirmed by the grinding effect in the MWCNTs.

Perspectives

For the first time, the specific heat C(T) of modified (milled and oxidized/milled) multi-walled carbon nanotubes (MWCNTs) with a diameter of 9.4 nm was investigated. The influence of the aspect ratio (length/diameter) was studied by comparing the data of initial CNTs and milled CNTs. The oxidation effect was studied by comparing milled and oxidized/milled CNTs with the same average length. These findings are crucial for the development and optimization of nanotechnologies utilizing carbon nanotubes. Funding This work was partly supported by the National Research Foundation of Ukraine (Grant 2023.03/0012).

Maksym Barabashko
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine

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This page is a summary of: Experimental evidence of flexural phonons in low-temperature heat capacity of carbon nanotubes, Carbon Trends, April 2025, Elsevier,
DOI: 10.1016/j.cartre.2025.100479.
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