Thermo-mechanical stability of a cellular assembly of carbon nanotubes in air

Abstract

Carbon nanotubes (CNT) in bulk form offer outstanding structural and functional properties, and are shown to remain viscoelastic over a wide temperature range (77–1273 K) under inert conditions. We examine the quasi-static and dynamic compressive mechanical response of these cellular CNT materials in ambient air up to a temperature of 773 K. In uniaxial quasi-static compression, several displacement bursts are noted at large strains. These are results of the slippage and zipping of the CNT, and lead to significant mechanical energy absorption. Results of the dynamic mechanical analysis experiments show no degradation in storage modulus and loss coefficient for up to 20 h at 673 K. Hence, these stable cellular CNT structures can be utilized up to a maximum temperature of 673 K in air, which is much higher than the best polymers.

Introduction

Carbon nanotubes (CNT) in bulk form have complex foam-like cellular microstructure and exhibit multi-scale organization [1], [2], [3], [4]. As a result, they reveal exceptional mechanical performance, especially high mechanical energy absorption during compression loading [1], [2], [3], [4], [5]. In addition to being lightweight, strong yet flexible, they offer outstanding thermal and electrical conductivities and electro-chemical properties [6], [7]. These combinations of properties make these cellular materials a suitable candidate for applications such as mechanical energy absorbers, heat exchangers, heat sinks etc., applications where multi-functionality is essential. Hitherto, metallic cellular materials (foams or honeycombs) are the most commonly used materials for such applications [8]. Although, polymer foams possess superior specific energy absorption characteristics, their applicability is limited to lower temperatures with some specific polymer composites having the ability to operate at a maximum temperature of ∼548 K [9] with certain limitations. The cellular assembly of CNT combine the characteristics of both metal and polymer materials and hence have been attracting great attention. Moreover, electrically conducting CNT structures, which can be fabricated with ease, allow for in situ monitoring of deformation, making them as ‘active materials’.

Electrical, mechanical, and thermal properties of aforementioned cellular CNT materials have been examined in detail and reported in literature [3], [10], [11]. In uniaxial compression, cellular CNT materials are viscoelastic, similar to polymers [1]. Recently, dynamic mechanical analysis (DMA) was employed to show that a randomly networked CNT film (bucky paper) exhibits rubber-like temperature- and frequency-invariant viscoelasticity for temperatures ranging between 77 and 1273 K [11]. For comparison, this temperature range is much narrower, 118–573 K, for rubber. However, these tests were performed in an inert atmosphere whereas many envisioned applications of these materials are going to be in ambient air. In such a case, oxidation of CNT is distinct possibility, which would reduce the utility temperature range of cellular assembly of CNT substantially. Further, mechanical loads may accelerate the oxidation-induced degradation of the mechanical performance of such structures. These aspects, which have not been examined hitherto, are studied in this paper where we perform DMA on macroscopically aligned CNT under uniaxial compression from 300 to 773 K in ambient air.