NEW CARBON MATERIALS Volume 29, Issue 3, June 2014 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2014, 29(3): 193–202. Received date: 20 February 2014; Revised date: 07 June 2014 *Corresponding author. E-mail: marcia@if.ufrgs.br; marciagallas@gmail.com Copyright©2014, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(14)60134-7 RESEARCH PAPER Preparation of carbon nanotube monoliths by high-pressure compaction Pamela Andréa Mantey dos Santos 1,3 , Ivana Zanella 1 , Tania Maria Haas Costa 2 , Patrícia Rodrigues da Silva 3 , Marcia Russman Gallas 3 1 Área de Ciências Tecnológicas, Centro Universitário Franciscano, UNIFRA, Santa Maria 97010-032, RS, Brasil; 2 Instituto de Química, Universidade Federal do Rio Grande do Sul, UFRGS, Porto Alegre, 91501-970, RS, Brasil; 3 Instituto de Física, Universidade Federal do Rio Grande do Sul, UFRGS, Porto Alegre, 91501-970, RS, Brasil Abstract: High-pressure compaction was used to produce monolithic multiwall carbon nanotubes (MWCNTs) from different sources: (1) high-purity commercial Baytubes®, (2) chemical-vapor deposited MWCNTs without purification at the Laboratory of Production of CNT/UNIFRA, and (3) the same MWCNTs as (2) purified with HCl. Pressures of 4.0 GPa and 7.7 GPa were applied at room temperature using two different pressure-transmitting media, lead and graphite. Cylindrical monolithic MWCNTs with diameters of about 6 mm were obtained. The samples were characterized by Raman spectroscopy, X-ray diffraction, elemental analysis, N2 adsorption and transmission electron microscopy. Results showed that the best sample was obtained with MWCNTs without purification, containing residues of MgO catalyst, and using lead as the pressure-transmitting medium at 7.7 GPa. High-pressure may cause compressive stress and shear stress for the MWCNTs. The lead container, as a quasi-hydrostatic pressure-transmitting medium, provided more compressive stress than shear stress while the impurities acted as binding materials. Both helped to obtain better densification of the MWCNTs. Key Words: Multiwall carbon nanotubes; High-pressure; Bulk MWCNTs samples; Raman spectroscopy; TEM 1 Introduction The discovery of carbon nanotubes (CNTs) has opened a new frontier in the chemistry and physics of carbon, attracting the attention of world-wide research since the early 1990s [1-3] . These cylindrical structures are formed by hexagonal arrays of carbon atoms, having a diameter between few angstroms to tens of nanometers and the length can be of the order of centimeters [1] . CNTs are unique structures with remarkable electronic and mechanical properties and have been extensively studied, aiming at the applications in several fields such as medical sciences [4-6] , electronics [7] , and composite materials [8-13] . During the last decade, ceramic matrice composites reinforced by CNTs have been studied, seeking to improve the intrinsic brittleness of these materials. One of the techniques used to produce ceramic compacts with CNTs as reinforcement is the high-pressure. Andrade et al. obtained dense compacts of silica/CNT with great tenacity (greater than 60% compared to silica without reinforcement) by this technique [12] . High-pressure (up to 8.0 GPa) has been used as an important tool for compaction and densification of nanometric powders, producing new materials. This process promotes the improvement of mechanical properties, allowing the production of hard and dense materials, optically transparent, and crack free [12,14-17] . In these works it was only possible to obtain such impressive properties using lead (Pb) containers as quasi-hydrostatic pressure-transmitting medium assembled in toroidal-type high-pressure chambers and making the processing at room temperature. Recently, the preparation of macroscopic CNTs has been investigated, here including sheets, fibers, pellets and films, targeting at exploring the important properties, such as mechanical and electrical, of the individual nanotubes. If these properties were preserved in macroscopic samples, they could present numerous applications in various systems such as solar cells, capacitors, electrodes, chemical sensors and others [18] . However, the actual performance of CNTs macroscopic samples is still far from the expectations, requiring a great deal of research yet. There are few studies in literature dealing with the preparation of self-supported CNTs bulk samples, i.e. a matrix formed only by CNTs, or with a very high content of CNTs. Some studies showed the preparation of pellets obtained from solutions of CNTs functionalized with HCl [19,20] . Cha et al. [18] produced other kind of samples prepared with CNTs interconnected via functional groups acquired during chemical treatment and plasma sintering. Xu et al. [21] prepared pellets by CVD method obtained from nanosized MgO powders, containing CNTs and