Radial thermal expansion of single-walled carbon nanotube bundles at low temperatures

Abstract

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].

Low-temperature radial thermal expansion of single-walled carbon nanotube bundles saturated with nitrogen

“…Thesample (a cylinder ~7.2 mm high and ~10 mm in diameter) was used to investigate the effect of nitrogen on the radial thermal expansion α r of SWNT bundles. (Previously this sample was used to measure α r of Xe-c-SWNT [1] and H 2 -c-SWNT [2] systems.) The measurements were made in the interval 2.2-43 K using a low temperature capacitance dilatometer with a sensitivity of 0.02 nm.…”

“…Nevertheless, some properties of carbon nanotubes (CNTs) and their bundles, the low temperature thermal expansion among them, still remain obscure. Dilatometric measurements of the radial thermal expansion coefficient α r on singlewalled carbon nanotube (SWNT) bundles at T = 2-120 K [1,2] show that α r is positive above 5.5 K and negative at lower temperatures. The negative α r values can be attributed to the low-frequency vibrations of individual nanotubes at liquid helium temperatures.…”

“…A controlled desorption of the impurities from the doped samples showed that the α r (T) peaks decreased with the impurity concentration. It was interesting to extend the investigations mentioned [1,2,4] and to find out how the radial thermal expansion of SWNT bundles could be influenced by gases consisting of nonspherical molecules. In this study the radial thermal expansion of N 2 -saturated SWNT bundles was measured in the temperature interval 2.2-43 K. To interpret the results obtained, we needed information about the concentration and the spatial distribution of the N 2 molecules in SWNT bundles.…”

Quantum effects in the radial thermal expansion of bundles of single-walled carbon nanotubes doped with He4

“…The radial thermal expansion of CNTs with closed ends saturated with 4 He was investigated on the sample that was previously used to obtain the radial thermal expansion coefficient of pure SWNTs and gas-saturated CNTs [1][2][3][4]. The measurements were performed using a low temperature capacitance dilatometer (its design and measurement technique are described in [7]).…”

“…Previously [1][2][3][4] we used the dilatometric method to investigate the low temperature radial thermal expansion of carbon nanotube (CNT) bundles and the effect of atomic (Xe) and molecular (H 2 , N 2 ) impurities on it. A number of effects have been detected, which are induced, on the one hand, by low frequency vibrations of individual CNTs and, on the other hand, by the collective behavior of the tubes forming a bundle.…”