Multi‐walled carbon nanotubes with ppm level of impurities

Abstract

Here multi-walled carbon nanotubes (MWNT) of different average diameters with ppm level purity were produced by a low temperature C 2 H 4 pyrolysis on Fe-Co type catalysts combined with forthcoming graphitization at 2200-2800 8C in argon flow. Annealed nanotubes were characterized with X-ray fluorescent analysis, BET surface measurements, HR TEM, X-ray diffraction, DTA analysis, and measurements of temperature and magnetic field dependences of conductivity. The graphitization of MWNT results in removal of residual catalyst metal impurities, reduction of the wall defects, and closure of nanotube tips. It was found that extent of these effects depends on MWNT diameters. It was proposed that the graphitization is caused by a significant thermal displacement and diffusion of carbon atoms at temperatures higher the Debye temperature. 1 Introduction To date, most carbon nanotubes synthesized via catalytic CVD method are applied for composite materials production. It should be mentioned that composite materials needed for high-temperature applications require pure carbon nanotubes because metal impurities can catalyze the composite destruction under these conditions. At the same time catalytic CVD method results in the encapsulation of metal particles inside the multi-walled carbon nanotubes (MWNT) core. Hightemperature treatments are traditionally used to produce high quality graphite and graphite-based materials [1][2][3]. Several research groups have investigated effects of MWNT high-temperature annealing at 1500-3000 8C [4][5][6]. It was shown that high-temperature treatment leads not only to the catalyst removing but also to the changes of tube structure and morphology [7,8].In this paper, the systematic study of influence of hightemperature treatment on MWNT structure and properties was performed. Two series of MWNT with different average diameters were used and characterized with various physical methods. It allows the technique of production of thin MWNT with ppm impurity level and with improved structure to be developed.

Fe–Mo and Co–Mo Catalysts with Varying Composition for Multi‐Walled Carbon Nanotube Growth

“…Our previous papers demonstrated the high sensitivity of Raman spectroscopy for estimation of the defective structure of MWCNTs and graphene flakes deposited on the nanotube surface . The graphene fragments have been proposed to form a mosaic structure of nanotube walls …”

Reinforcement of CVD grown multi-walled carbon nanotubes by high temperature annealing

“…26 We have applied better control of the exchange gas and used better pristine CVD nanotubes than in previous studies. Figure 1 shows the TEM images of a large assembly of pristine CVD-MWNTs pristine and heat treated at 2800 o C (the highest temperature used in this work).…”

“…22,23 A large scale healing of the structural defects and the consequent increase of the Young's modulus is of paramount importance for the application of MWNTs as reinforcement fibers in composites. Here, we report a systematic improvement of the mechanical strength of MWNTs grown using the multicomponent catalyst with Fe-Co active component 24,25 and annealed in pure argon flow at 2200-2800 o C. 26 The contrast to the previous studies, 21 where no considerable reinforcement was detected, is ascribed to a better control of the heat treatment procedure and to the much better quality of the pristine tubes grown on the alloyed Fe-Co nanoparticles forming in-situ under ethylene flow. 25 Today, for MWCNTs synthesis, different improved catalyst preparation methods are developed 27 and the carbon nanotube batches produced can be more uniform.…”

Direct Vapor-Phase Bromination of Multiwall Carbon Nanotubes

“…• C, see [13] for details) were performed. Annealed nanotubes possess perfect structure of graphene sheets and very low amount of structural and topological defects; thus this sample was used as comparative sample for elucidation of bromination mechanism.…”