Integrated random-aligned carbon nanotube layers: deformation mechanism under compression† Zhiping Zeng, a Xuchun Gui, * a Qiming Gan, a Zhiqiang Lin, a Yuan Zhu, a Wenhui Zhang, a Rong Xiang, a Anyuan Cao b and Zikang Tang ac Carbon nanotubes have the potential to construct highly compressible and elastic macroscopic structures such as films, aerogels and sponges. The structure-related deformation mechanism determines the mechanical behavior of those structures and niche applications. Here, we show a novel strategy to integrate aligned and random nanotube layers and reveal their deformation mechanism under uniaxial compression with a large range of strain and cyclic testing. Integrated nanotube layers deform sequentially with different mechanisms due to the distinct morphology of each layer. While the aligned layer forms buckles under compression, nanotubes in the random layer tend to be parallel and form bundles, resulting in the integration of quite different properties (strength and stiffness) and correspondingly distinct plateau regions in the stress–strain curves. Our results indicate a great promise of constructing hierarchical carbon nanotube structures with tailored energy absorption properties, for applications such as cushioning and buffering layers in microelectromechanical systems. Introduction Demand is increasing for porous nanomaterials that can absorb and dissipate energy for civil and military applications. 1 In recent years, a variety of nanoporous materials have been developed for use as light-weight energy dissipation materials, such as ultralight metallic microlattices and nanosilica, 2,3 molecularly intercalated nanoakes, 4,5 periodic bicontinuous composites, 6–9 and carbon nanomaterials. 10–14 Selection of appropriate materials and design of the geometrical structure are two effective approaches to construct energy dissipation materials with high mass- or volume-specic energy absorption. 6,15 Carbon nanotubes (CNTs) are widely used as the basic units to assemble macroscopic materials with low density, high porosity, and excellent compression capability, with potential applications as energy absorption and protective materials. 16–18 Previous investigations have indicated that vertically aligned CNT arrays exhibit an anisotropic structure, and unique response under compression along their length. 19 Under external impact, the CNT arrays dissipate energy by structural buckling and friction between buckled CNTs. 20 The mechanical response of CNT arrays under impact or cyclic compression is directly related to the deformation of microscopic structure. 21 Interestingly, in situ observations have shown that CNT arrays deform by collectively forming a buckled structure. 11,20–23 Recently, porous isotropic CNT sponges with high structural exibility and robustness have been reported by our group. 16 These sponges can tolerate large compressive strains repeatedly without collapse, and exhibit high energy dissipation ability. 24,25 A typical geometrical structure design for protective mate- rials is to adopt layered structures combining hard and so materials for tailored strength and toughness. 6 This approach has proven to be effective in the design of energy dissipation systems, for example, multilayer structures consisting of alter- nating layers of aligned CNT arrays and metal foil, 26 CNT arrays and vermiculite; 17 CNT arrays and polymers have been devel- oped to form highly efficient vibration and energy dampers. 15,27 Although a number of CNT-based energy absorption materials have been reported, they usually contain other inorganic or polymeric components that are brittle or unstable at high temperature. Recently, we reported a well-dened all-CNT tandem composite consisting of an aligned (array) and a random (sponge) layer connected in series, with a wider range characterized by relatively low cushioning coefficients compared to individual arrays or sponges. 28 a State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China. E-mail: guixch@mail.sysu.edu.cn b Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China c Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China † Electronic supplementary information (ESI) available: Low-magnication SEM images showing the deformation process of the double-layered structure, SEM images of sponge–array double-layered structure under the compressive strain of 32% and 52%, and compressive stress–strain curves of a CNT array and sponges separately for 50 cycles. See DOI: 10.1039/c3nr04667b Cite this: DOI: 10.1039/c3nr04667b Received 2nd September 2013 Accepted 24th November 2013 DOI: 10.1039/c3nr04667b www.rsc.org/nanoscale This journal is © The Royal Society of Chemistry 2014 Nanoscale Nanoscale PAPER Published on 27 November 2013. Downloaded by Nanyang Technological University on 11/01/2014 08:58:52. View Article Online View Journal