The radial thermal expansion coefficient α r of pure and Xe-saturated bundles of singlewalled carbon nanotubes has been measured in the interval 2.2-120 K. The coefficient is positive above T = 5.5 K and negative at lower temperatures. The experiment was made using a low temperature capacitance dilatometer with a sensitivity of 2·10 -9 cm and the sample was prepared by compacting a CNT powder such that the pressure applied oriented the nanotube axes perpendicular to the axis of the cylindrical sample. The data show that individual nanotubes have a negative thermal expansion while the solid compacted material has a positive expansion coefficient due to expansion of the intertube volume in the bundles. Doping the nanotubes with Xe caused a sharp increase in the magnitude of α r in the whole range of temperatures used, and a peak in the dependence α r (T) in the interval 50-65 K. A subsequent decrease in the Xe concentration lowered the peak considerably but had little effect on the thermal expansion coefficient of the sample outside the region of the peak. The features revealed have been explained qualitatively.
Abstract.The linear coefficient of the radial thermal expansion has been measured on a system of SWNT bundles in an interval of 2.2 -120K. The measurement was performed using a dilatometer with a sensitivity of 2⋅10 -9 cm. The cylindrical sample 7 mm high and 10 mm in diameter was obtained by compressing powder. The resulting bundles of the nanotubes were oriented perpendicular to the sample axis. The starting powder contained over 90% of SWNTs with the outer diameter 1.1 nm, the length varying within 5-30 μm.Since the discovery of carbon nanotubes in 1991 [1], this novel class of physical objects has been attracting immense experimental and theoretical interest. However, a wide variety of types of carbon nanotubes and the problems encountered in preparing pure samples of these types of nanotubes makes it extremely difficult to reveal regularities in the behavior of nanotubes (e.g., see [2] and the References in it). The thermal expansion of nanotubes remains among the least studied properties of carbon nanotubes. For example, the thermal expansion of single-walled nanotubes and their bundles has never been investigated experimentally below room temperature. Meanwhile, investigations at low temperatures can provide the most valuable information about the dynamics of nanotubes. The thermal expansion coefficients (TECs) predicted theoretically [3 -8] for singlewalled nanotubes (SWNT) differ in order of magnitude and sign.In this study the radial thermal expansion of bundles of closed end SWNTs (c-SWNT) was measured in an interval of 2.2 -120 K.The sample for measuring thermal expansion was prepared using the effect of aligning the SWNT axes by pressure of 1 GPa (see [9]). Under this pressure the nanotubes within a 0.4 mm thick layer are aligned in the plane normal to the pressure vector, the average deviation from the plane of alignment being ~ 4° [9].
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